You can have a look at how the structure looks at this link – https://developer.wordpress.org/reference/functions/wp_nav_menu/
You can add your classes into those parameters and it should work. Have a look at examples like the themes that come standard with WordPress and use the Inspect tool on Chrome to see the classes that get added.
Basically you will have to play with the classes and other parameters to make it work dynamically. See this link for an example – http://www.yogihosting.com/integrate-bootstrap-menu-wordpress-website/
If you want a dropdown you will need to read up on how to implement the nav walker to the code. It will allow you to make a dynamic dropdown from the menu you create in WordPress backend. You’ll just have to style the dropdown to fit your menu. You can download the nav walker code here – https://github.com/wp-bootstrap/wp-bootstrap-navwalker