More precisely: File/Library/Open Library
If you have several libraries, you can 'always prompt for library selection when MB starts up'.
(If you have this option unchecked, you can use the shortcut Psychoadept suggested.)
When you create a Library, you choose a folder and in that folder, you will find the files you're looking for (I guess).
Once created, you can only move that library - indeed.
Other files are either in Program Files/MusicBee or in AppData/Roaming/MusicBee (Windows).
b.