It's easy to make a multi-level drop-down menu in Divi. First, you can go to WordPress Dashboard > Appearance > Menus in your WordPress dashboard and select the menu items you want to add to the menu.
To make a menu item a child of another menu item, simply drag it slightly to the right under the parent menu item.
You will find that it stays there. Once it's on the right, it will appear in a drop-down menu on the front end of the website. So, in the gif posted above, the menu items "Fixed Button, A Link, and Contact Form" will appear in a drop-down menu.
You can add more drop-down levels in the same way.
Bonus: If you want to create a mega menu with Divi, then the following article can help you to achieve that: Creating Mega Menus In Divi