I dragged my music collection from my WIN10 PC (explorer) which had many folders and sub-folders, into the MUSIC tab of MusicBee. I did not realize that was a shortcut/command to take the files and physically move them into the WIN10 allocated Music folder (which every PC has), not only moving my collection but deleting the file system. This is not behavior I'd expect from a desktop program.
Doing what you described would not have moved your music files anywhere (unless you specifically set MusicBee up to do that before hand under Edit->Edit Preferences->Library...music library...auto-organize media files).
The Music node in Windows Explorer is not an actual folder on your system. It coaltes all the places on your drive where music already sits under one heading.
I know now I should have clicked on the + sign and used the shortcuts on the left; but the Music tab seemed like the place to start.
No, that's not the way to import music.
The way to import music is to go to File->Scan Folders For New Files and then point MB to where your music is.
So what I am looking for now is, to find out what Musicbee did with that file system and if it can be restored, before I embark upon redoing it all.
I'm not sure why you keep saying "File System" in this context. It doesn't mean what you think it means, and confuses the issue for people who assume that you're using it in the correct way.
If you only did what you described, I'd expect your music files to be exactly where they started.
If you go into your MB library in Tracks view and add the "Path" field to "Select Displayed Fields" by right-clicking the column headers,it will show you where each file is on your drive.