How can I customize a nav bar that only displays on mobile?

Hi there,

what are you trying to do specifically on the mobile navigation?
Does the Mobile Header option cover that need:

https://docs.generatepress.com/article/mobile-header/

I want to achieve something like this. Where there is a menu up top on mobile, but there's also a drop down menu beneath on the left.
https://i.imgur.com/51YtlIZ.png

Simplest method is to add a Navigation Menu Widget to the Top Bar Widget area.
Then use this CSS to hide it on tablet / desktop:

 @media (min-width: 420px) {
     .top-bar {
         display: none;
     }
 }

The Primary Navigation can set to be below header. That covers the basics i think