I see that there is documentation for creating a module in C++, but it’s not clear how to write a GDScript library that can be used on the Godot Asset Store. Is there a code structure or directory layout that’s required? What’s the API for connecting to people’s projects when they download the code asset?
“Modules” as described on that page are not on the Godot Asset Store, they are to be compiled with the Godot engine.
Most Godot Asset Store packages are GDScript libraries. You upload your asset as a zip folder, the store publishing page will tell you if anything is wrong before it goes public