Is there any way to customize the submenu?

I would like to arrange the array horizontally, and I would like to have long boxes appear in a row in the background when a submenu is opened. Is there any customization for submenus?

There are design and structure limitations in Wix’s menu. If you want a richer menu, you have to build the components yourself.
If our client wants a unique menu, we usually develop it in blocks and install the application on the website
Using multi box and repeaters in combination with cms enables the ability to produce menus of all types.
all small - Navigation Menu
Visit our website for information on:
https://www.cmsblocks.com/

Take a look here…
https://dev.wix.com/docs/velo/api-reference/$w/menu/css-classes