Much of the code for any page, including header and footer, comes from your theme.
If you want to create fully custom code, your best bet is to create a child theme (which just means creating a “style.css” file with some comments that refer to the parent theme, which is what you’re currently using) and inside the child theme, create custom templates. It’s there where you can add your own HTML and a loop to grab the content for each page (the part you edit in wp-admin).