Working with Child Theme folder structure

Hello, my question/dilemma; aside from customizing css and php at the Appearance>Editor and various other places on the Dashboard, I’ve never fully understood customizing the folder structure of a Child Theme, even when doing searches my questions wasn’t really address or I didn’t find a good tutorial.

I have someone that helps with coding, however I would like to know the process so that I can proper back up and also use what he does on a second site with the same theme here that I use.

If I add or change code in a php file that resides in a particular folder structure do I copy the same into the Child Theme?

example:

Main Theme structure:

Folder>Folder1>Folder2>blank.php

Copy same Folder structure into Child Theme:

Folder>Folder1>Folder2>blank.php (with added custom code)

If that is the case my other question is, do I copy the entire php into the Child Theme, all of its code as well as my changes into the Child Theme as well? Or do I just use the snippet of code that I added or change?

Thanks I really hope to have this cleared up in my head.