Bismark I agree that the UI is confusing in that the side menu shows a menu entry called Documents, which doesn't take you to the Documents folder located in your home folder, but rather displays a list of folders which contain what it considers document files (likely based on looking for certain file extensions, but I haven't looked at the code). I was confused by this when I first started using the app. The Google Files app is slightly more clear on that point but it's also not ideal.
To add to the confusion, the Downloads menu entry does take you to the Downloads folder. I think the side menu ought to be more clearly categorised. The AOSP apps are in the process of being overhauled and replaced, so perhaps changes will be made to the Files app at some point?
Bismark While backing up my files from my Documents folder on my pixel I moved everything out of the folder and onto my PC. According to Dolphin file manager in Linux the Document folder on pixel is now empty, but according to the Files app (com.android.documentsui) it says there are still files there.
If you moved files from the Documents folder to another folder, the Documents folder should be empty. Perhaps the Files app is caching an old file list. Bit strange.