GI1
dirksche Agreed, so I tried to figure it out.
I played with around with Ghost Commander, and found that the normal access to all files permission you can grant using Android's UI doesn't give it access to a storage provider (like your NAS or cloud storage that show up in the Files app).
Looking around Android developer docs, I found that storage providers' data can only be accessed using Storage Access Framework. That means that if you want to access a folder or file on your NAS or cloud storage using some app, that app will use the document picker to have you browse to the folder/file you want to grant temporary access to.
Traditional "all files access" is covered on this page. Even though the permission is called MANAGE_EXTERNAL_STORAGE, it really just gives an app access to "internal" storage, including devices that were mounted by the system like a USB drive or SD card.