There is no HTML code for the layout of the quiz. There is PHP code that generates the HTML, as it is requested.
What you are requesting looks like a fairly significant change to the layout of the quiz, so it's not something that can be quickly fixed in a forum post here.
As a starting point, you can look at the function attempt_page() in mod/quiz/renderer.php. Once you've traced through the function calls from that, you should be able to adjust your theme to override the functions in this renderer and output a custom layout: https://docs.moodle.org/dev/Overriding_a_renderer