please help me better understand the template override system I've been making tons o

Hello.
I am new to this so bear with me.
I made my "custom" folder and set it as my template in admin etc...
I also made a few matching "custom" folders in a few places where I saw a classic folders.

So then I went in and made a bunch of changes through my admin. Now I'm wondering if I am altering the .php files in "template_default" or am I altering the ones in my "custom" template????

So then let's assume that I'm altering the ones in template_default and so I am aware that I need to copy those files to my 'custom' folder however how do I know where those .php files reside?

For example I just changed one of my email settings for new customers through ADMIN. So assuming that I just altered the .php file in template_default, how do I know where to go to find that file in order to copy it?

Do I even need to?

With the exception of the Defined pages, any alterations that you make through the Admin are Database settings, not file changes.

That of course assuming you did not add any third party tools or contribs. :)

Cheers.