How to push data into 2D arrays at specific positions?

You can only "push" onto the next free position.

But you can assign to any position. For example:

var x = [];                 // create array "x"
for(let i = 0; i < 3; i++)  // x has 3 inner arrays
  x[i] = [];
x[2][4] = 'test';           // assign to any desired position
x[1][2] = 'this';

If you need help with some code you can always create a question and post it here. Be sure to format your code using the instructions in the Markdown Cheatsheet link at the bottom of the "Add an Answer" section.

For the question, "How/where can I paste code for review," you can paste it here. I don't know if this is the same for you, but the 'Markdown Cheatsheet' linked below the 'Add an Answer' section for me links to the community instead of a helpful formatting sheet. If this is the case for you as well, this link is for Posting Code and here's a Markdown Guide.