[sciurius]

If I make a change to a song or add an annotation or add, for instance, a genre to a song, if I have it in multiple databases how will all copies of that song's characteristics be updated in each database?

They will not, and that is exactly the purpose.
As I see it: When you update a song file on disk, and that song is external to MSPro (not managed) and also used in another databases, switching to that other database will pick up the changes. This is no different from otherwise changing an external song file.
However, adding annotations, notes, etc, to a song applies to the current database only. I may, and will, have different annotations for the same song, depending on whether it is used in (eg.) a choir or a cover group.

The semantics would be identical to making a backup set of DB1 and then restore the backup set (with total clean) of DB2. Anything not physically in the backup set is essentially shared, everything else is private to this database.