Q:

How to populate an array within an array in MATLAB and how to access them?

For example
A = {{1,2},{23,34},{45,4},...}

How to create such a data type in MATLAB
How to access the i-th element and the elements within it?
For Ex. A[2] should return {23,34} and A[2].1 should return 23.

A:

A = {{1,2},{23,34},{45,4},...}

is valid MATLAB syntax, if you are trying to make a cell array of cell arrays. However, you'd probably want to store vectors rather than arrays in your array:
A = {[1,2],[23,34],[45,4],...}

Access them like A{2} or A(2) but far more likely the first one. If you want an individual element then A{2}(1)
But if every one of your cells is going to contain a 2 element vector then you will have a much easier time just using a 2D matrix:
A = [1,2;23,34;45,4;,...]

And and now access the entire row i.e. A(2,:) or for an individual element A(2,1)

