Wild guess which might explain PART of what you're seeing:
Photos likes to store pictures within its own database, rather than "loose" in the Finder.
If you look in your Pictures directory, you may see a file called "Photos Library.photoslibrary" (or something like that). If you select it, and then right-click on it, the popup menu will include the item "Show Package Contents".
That means that this file is really a directory tree that the Finder is disguising as a single file – to discourage you from messing around inside, and destroying relationships between its components which the Photos application may be counting upon.
If you select Show Package Contents, the Finder will open a window onto the top-level folder of the package, and you can then navigate around and see things inside. If you do this, I would strongly recommend just looking – not touching. Don't move, delete, or modify anything. The internal structure may be organized in a what seems like a haphazard way - basically, because it only has to be convenient for Photos, not for you. The developers of Photos expect that if you want to export photos, you'll go use the File > Export menu item in the application.
I believe that Apple may have used packages for the iTunes Library in the past, but that these days the Music and TV applications store your imported media in folders. So I don't know why "nothing" would show up for them. It's also a good idea to apply a "look but don't touch" rule to these databases, whether they are in packages, or "just" in ordinary directories.