Q:

My profiler toolbar isn't showing up in symfony 4.3.1

In my .env file, I have specified my app environment to be dev and debug to be true like so:
APP_ENV=dev
APP_DEBUG=true

In my config/packages/dev/web_profiler.yaml file I have the following:
web_profiler:
    toolbar: true
    intercept_redirects: false

framework:
    profiler: { only_exceptions: false }

The routing within config/routes/dev/web_profiler.yaml seems to be fine:
web_profiler_wdt:
    resource: '@WebProfilerBundle/Resources/config/routing/wdt.xml'
    prefix: /_wdt

web_profiler_profiler:
    resource: '@WebProfilerBundle/Resources/config/routing/profiler.xml'
    prefix: /_profiler

So when I run the server with symfony server:start everything is fine, but the profiler doesn't appear. Did I miss something that enables that feature within Symfony?
To clarify, the page is outputting a proper HTML page with the appropriate content. There is just no profiler showing up.

My base twig template:
<!DOCTYPE html>
<html lang="en" dir="ltr">
    <head>
        <meta charset="utf-8">
        <title>{% block title %} {% endblock %}</title>
        {{ encore_entry_script_tags('base') }}
        <link rel="icon" type="image/x-icon" href="{{ asset('build/images/favicon.ico') }}" />
        <link href="https://fonts.googleapis.com/css?family=IBM+Plex+Sans:400,500|Playfair+Display:400,700&display=swap" rel="stylesheet">
        {{ encore_entry_link_tags("base") }}
        {% block stylesheet %}{% endblock %}
    </head>
    <body {% if app.request.get('_route') == 'home' %} class='homepage' {% endif %} >
        <header>
            <div id='top-navigation' class='padding-lg__left-md padding-lg__right-md padding-lg__top-sm padding-lg__bottom-sm row row__align-center row__justify-start'>
                <span class='text-color__white text-size__small text-weight__bold margin-lg__right-lg'>Our Mission</span>
                <span class='text-color__white text-size__small text-weight__bold margin-lg__right-lg'>Our Team</span>
                <span class='text-color__white text-size__small text-weight__bold margin-lg__right-lg'>Where the Money Goes</span>
                <span class='text-color__white text-size__small text-weight__bold margin-lg__right-lg'>Community Leadership</span>
                <span class='text-color__white text-size__small text-weight__bold'>Policies</span>
                <span class='text-color__white text-size__small text-weight__bold margin-lg__left-auto icon-set'> <span class='icon size__small color__white margin-lg__right-xsm'>{{ source('@public_path'~asset('build/images/icons/feedback.svg')) }}</span>Submit Feedback</span>
            </div>
            <nav class="padding-lg__top-md padding-lg__bottom-md padding-lg__left-md padding-lg__right-md row row__align-center row__justify-start {% if app.request.get('_route') == 'home' %} homepage {% endif %}">
                <div id='logo'>
                    <a href="{{ url('home') }}">
                        <img src="{{ asset('build/images/logo_placeholder.png') }}" alt="logo">
                    </a>
                </div>
                {% if app.request.get('_route') == 'creator-register' %}

                {% else %}
                    {% if not is_granted('IS_AUTHENTICATED_FULLY') %}
                        <div class='margin-lg__left-auto'>
                            <a href="{{ url('login') }}">
                                <div class='icon-set'>
                                    <span class='icon margin-lg__right-xsm'>
                                        {{ source('@public_path'~asset('build/images/icons/user.svg')) }}
                                    </span>
                                    <span class='nav-item'>Login</span>
                                </div>
                            </a>
                        </div>
                    {% endif %}

                {% endif %}
            </nav>
        </header>
        {% if app.request.get('_route') != 'home' %} <div class='container is_top'> {% endif %}
            {% block body %} {% endblock %}
        {% if app.request.get('_route') != 'home' %} </div> {% endif %}
    </body>
</html>

Security.yaml firewall:
    firewalls:
            dev:
                pattern: ^/(_(profiler|wdt)|css|images|js)/
                security: false
            main:
                anonymous: true
                guard:
                    authenticators:
                        - App\Security\LoginFormAuthenticator
                logout:
                    path : logout
                remember_me:
                    secret: '%kernel.secret%'
                    lifetime: 2592000 #<- 30 days in seconds - defaults to one year if you take this out!

Results on php bin/console debug:router | grep _profiler:
  _profiler_home             ANY      ANY      ANY    /_profiler/                        
  _profiler_search           ANY      ANY      ANY    /_profiler/search                  
  _profiler_search_bar       ANY      ANY      ANY    /_profiler/search_bar              
  _profiler_phpinfo          ANY      ANY      ANY    /_profiler/phpinfo                 
  _profiler_search_results   ANY      ANY      ANY    /_profiler/{token}/search/results  
  _profiler_open_file        ANY      ANY      ANY    /_profiler/open                    
  _profiler                  ANY      ANY      ANY    /_profiler/{token}                 
  _profiler_router           ANY      ANY      ANY    /_profiler/{token}/router          
  _profiler_exception        ANY      ANY      ANY    /_profiler/{token}/exception       
  _profiler_exception_css    ANY      ANY      ANY    /_profiler/{token}/exception.css 

Lastly homepage controller:
<?php
namespace App\Controller;

use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\Routing\Annotation\Route;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;

class HomepageController extends AbstractController{

    /**
    * @Route("/", name="home")
    */

    public function output(){
        return $this->render('homepage/home.html.twig',[
            'title' => 'yo',
        ]);
    }
}

?>

Added public/index.php:
<?php

use App\Kernel;
use Symfony\Component\Debug\Debug;
use Symfony\Component\HttpFoundation\Request;

require dirname(__DIR__).'/config/bootstrap.php';

if ($_SERVER['APP_DEBUG']) {
    umask(0000);

    Debug::enable();
}

if ($trustedProxies = $_SERVER['TRUSTED_PROXIES'] ?? $_ENV['TRUSTED_PROXIES'] ?? false) {
    Request::setTrustedProxies(explode(',', $trustedProxies), Request::HEADER_X_FORWARDED_ALL ^ Request::HEADER_X_FORWARDED_HOST);
}

if ($trustedHosts = $_SERVER['TRUSTED_HOSTS'] ?? $_ENV['TRUSTED_HOSTS'] ?? false) {
    Request::setTrustedHosts([$trustedHosts]);
}

$kernel = new Kernel($_SERVER['APP_ENV'], (bool) $_SERVER['APP_DEBUG']);
$request = Request::createFromGlobals();
$response = $kernel->handle($request);
$response->send();
$kernel->terminate($request, $response);

A:

It's very hard, if not impossible, to debug this for you remotely. The exact problem is tied to something specific in your local setup, and someone without access to your project would not have a chance to see exactly what is wrong.
Some general and specific troubleshooting advice for your situation:
1st. Reinstall the profiler pack
While unusual, the installation could be borked. Make sure your profiler package is alright.
First remove it (composer remove profiler), and then install it again: composer require --dev profiler).
2nd. Check the configuration
Use Symfony's console command to verify your configuration.
First for the built-in profiler:
$ bin/console debug:config framework profiler

Which should return something like this:
Current configuration for "framework.profiler"
==============================================

only_exceptions: false
enabled: true
collect: true
only_master_requests: false
dsn: 'file:%kernel.cache_dir%/profiler'

And then for the profiler toolbar:
$ bin/console debug:config web_profiler

Which should return something like:
Current configuration for extension with alias "web_profiler"
=============================================================

web_profiler:
    toolbar: true
    intercept_redirects: false
    excluded_ajax_paths: '^/((index|app(_[\w]+)?)\.php/)?_wdt'

3rd. Check the container
Check how the Profiler service will be instantiated:
$ bin/console debug:container profiler --show-arguments

Expect something like this:
Information for Service "profiler"
==================================

 Profiler.

 ---------------- -------------------------------------------------------------------------------------
  Option           Value
 ---------------- -------------------------------------------------------------------------------------
  Service ID       profiler
  Class            Symfony\Component\HttpKernel\Profiler\Profiler
  Tags             monolog.logger (channel: profiler)
                   kernel.reset (method: reset)
  Calls            add, add, add, add, add, add, add, add, add, add, add, add, add, add, add, add, add
  Public           yes
  Synthetic        no
  Lazy             no
  Shared           yes
  Abstract         no
  Autowired        no
  Autoconfigured   no
  Arguments        Service(profiler.storage)
                   Service(monolog.logger.profiler)
                   1
 ---------------- -------------------------------------------------------------------------------------

And then for the web_toolbar:
# bin/console debug:container web_profiler.debug_toolbar --show-arguments

For something like this:
Information for Service "web_profiler.debug_toolbar"
====================================================

 WebDebugToolbarListener injects the Web Debug Toolbar.

 ---------------- ------------------------------------------------------------------------
  Option           Value
 ---------------- ------------------------------------------------------------------------
  Service ID       web_profiler.debug_toolbar
  Class            Symfony\Bundle\WebProfilerBundle\EventListener\WebDebugToolbarListener
  Tags             kernel.event_subscriber
                   container.hot_path
  Public           no
  Synthetic        no
  Lazy             no
  Shared           yes
  Abstract         no
  Autowired        no
  Autoconfigured   no
  Arguments        Service(twig)

                   2
                   Service(router.default)
                   ^/((index|app(_[\w]+)?)\.php/)?_wdt
                   Service(web_profiler.csp.handler)
 ---------------- ------------------------------------------------------------------------

(Note the 2, which enables the toolbar).
4th. Check the event dispatcher.
The web debug toolbar is injected during the kernel.response event. Check that the callback is appropriately hooked:
$ bin/console debug:event-dispatcher kernel.response

Which will return something like this:
Registered Listeners for "kernel.response" Event
================================================

 ------- -------------------------------------------------------------------------------------------- ----------
  Order   Callable                                                                                     Priority
 ------- -------------------------------------------------------------------------------------------- ----------
  #1      ApiPlatform\Core\Hydra\EventListener\AddLinkHeaderListener::onKernelResponse()               0
  #2      Symfony\Component\HttpKernel\EventListener\ResponseListener::onKernelResponse()              0
  #3      Symfony\Component\HttpKernel\DataCollector\RequestDataCollector::onKernelResponse()          0
  #4      Symfony\Component\WebLink\EventListener\AddLinkHeaderListener::onKernelResponse()            0
  #5      Symfony\Component\Security\Http\RememberMe\ResponseListener::onKernelResponse()              0
  #6      ApiPlatform\Core\HttpCache\EventListener\AddHeadersListener::onKernelResponse()              -1
  #7      Symfony\Component\HttpKernel\EventListener\ProfilerListener::onKernelResponse()              -100
  #8      Symfony\Bundle\WebProfilerBundle\EventListener\WebDebugToolbarListener::onKernelResponse()   -128
  #9      Symfony\Component\HttpKernel\EventListener\TestSessionListener::onKernelResponse()           -128
  #10     Symfony\Component\HttpKernel\EventListener\DisallowRobotsIndexingListener::onResponse()      -255
  #11     Symfony\Component\HttpKernel\EventListener\SessionListener::onKernelResponse()               -1000
  #12     Symfony\Component\HttpKernel\EventListener\StreamedResponseListener::onKernelResponse()      -1024
 ------- -------------------------------------------------------------------------------------------- ----------

Notice item #7, which is the Profiler collector (which among other things will include the X-Debug-Token header in the response, which will be later checked by the Web Debug Toolbar, which is item #8 in the above listing.
If any of the above checks fails
You'll have to focus on that specific part to find out why it is failing. Maybe some other bundle interfering? A problem with one of the configuration files?
Everything checks out
... but still not working? Well, that's weird. Make sure that your returned template has a </body> tag, and that the returned response has text/html content-type. But if all of the above checks out... it should work.

In a comment you say that framework.profiler.collect is set to false when performing these checks.
Set it to true by changing config/packages/dev/web_profiler.yaml like this:
framework:
    profiler:
        only_exceptions: false
        collect: true

