Where is the HTML/PHP code located, so we can make changes to the existing code?

There is no access to the underlying code to change things. You can add to functionality by creating applications or adding theme hooks, depending on what you are trying to achieve.

What if we were trying to add a button that automatically goes to the top/bottom of the page? Is this just done to each individual theme through the custom css in the theme editor?

Yes, that would be correct. Either by CSS additions, theme hooks, or adding JS items

Thanks

You can find a free application that adds a back to top button here: https://invision-market.com/apps/members/go-to-top-button-r127/

32 minutes ago, teraßyte said:

You can find a free application that adds a back to top button here: https://invision-market.com/apps/members/go-to-top-button-r127/

Great, thank you!

is it possible to insert code above </head>

if so, is there a guide how to do this? just installed 5 to try out before changing over our sites.

You can indeed. You would add a template hook to the globalHeader template

totally get it now

Actually think this is a good idea as i will have access to all my changes in one place. Name structure for my edits should make it much easier to access.

29 minutes ago, loccom said:

totally get it now

Actually think this is a good idea as i will have access to all my changes in one place. Name structure for my edits should make it much easier to access.

Glad you have what you need :)