I have completed the Learn Next course, and I am concerned that it created files such as lib and ui that are not related to Routing directly under the app directory.

Personally, I think it is best practice to place in app only the folder that will be the routing path, files with special names that affect the behavior of the App Router, and co-locations of one off files used on specific page.tsx that start with an underscore _foo.
I believe that this is the best practice.

I would be happy to share your thoughts!