How can I see the components (HTML, JS, PHP) of a wordpress page?

I've taken over a WordPress project after the previous "developer" (looking at its code, it's hard to call it that) has bothered the client.

I do not have much experience in WordPress, how could I discover how the previous developer built a page? I know that if I go to Pages> (Page that I want to edit) I can see certain parts of the page and that the Attributes section of the page tells me which template is using the page.

But this does not seem to show all the HTML and JavaScript. Where would I look for this?