ItayXD

Do I make an exact copy of the parent’s php file and copy it to the child theme and edit?

Yes

Does this mean the entire php template is being overridden when I do this?

Yes

No! That is not the right way to do that, as then you’ll have problems as you described:

OK, just wanted to make sure I was doing that correctly (as I couldn’t imagine a way where it could work like it does with CSS overrides).

So, let’s say I make a change to footer.php (via a child theme) and in a few months the parent theme is updated. What happens if the theme update makes a change to footer.php (parent theme) that affects the functionality of the updated theme? Since my, now old, footer.php (child theme) is out-of-date, the theme could break.

What’s the best way to deal with this (aside from testing theme updates in a staging environment before rolling out to live/production)?

Usually you use something called “hooks”, you can read about them here (best I found):
https://wp.smashingmagazine.com/2011/10/07/definitive-guide-wordpress-hooks/
https://wpcandy.com/teaches/how-to-use-wordpress-hooks/
https://wp.tutsplus.com/tutorials/the-beginners-guide-to-wordpress-actions-and-filters/
https://wp.tutsplus.com/tutorials/plugins/reel-em-in-understanding-hooks-from-the-inside-out/

have you checked your .htaccess? check what file is defined as the 404 page.

You should just open the zip file on your PC. You probably using windows so you can try 7z or winrar.

Indeed

Bump

I also tried to get the html of the header and footer of WP inside phpBB’s overallfooter and overallheader but it was kind of messed up I guess it because of css conflict. I know WP-united had a way to solve it, how did they do it? any other way to solve the css conflicts?