How do I modify the HTML of my subtheme? I created a bootstrap subtheme ouf of CDN. Modifying the CSS works, but how do I do it with the HTML? Where do I put new HTML elements? For example: I'd like to add some elements to my footer, make it background color black and have it the width of the screen (it's not by default in Bootstrap)

This is my subtheme folder:
A sub theme inherits its templates from the base theme. When you want to edit templates for your sub theme you simply copy over the desired template from the base theme's template folder into your sub theme's template folder, flush the cache and start editing it.

And then flush the cache again.