如何在Wordpress中找到UI菜单文件

in order to solve a dev. menu code problem, as the post title tell, I am struggling to find UI menu files in Wordpress. As you can see, (and please have a look at this link) there is a code implemented into the header.php file that should offer some pointers on where the current menu UI is held (unfortunately not exactly yet), outputting “menu1” and “menu2”;

At this point, I'm not so sure where are these located, in order to analyse/modify the UI menu files.

I know that generally these are held under wp-content folder, but where more exactly? because I can't find them.

In function.php I've found this

...
// Add Menu Support 
require_once('inc/wp3menu.php'); 
...

nothing more about menu1 or menu2 sections ...Also in wp3menu.php the same story (true there is Walker_Nav_Menu but no menu1 / 2 sections).

I know that names and links are stored in the database, but the UI files will be housed in files somewhere in the code base; is this available in every WP theme?

I appreciate it, Thanks.