I don’t use the database addons myself, but whether or not it is included in the backups depends on it’s storage container.
I know that the database of the pi-hole plugin is included in the addon snapshot, since it grew to big on me.
What you can always do is try installing, create a snapshot and open and browse the contents.
Note: creating backups from an open database is not a good plan. You have open files and risk a corrupt backup. So always take specific measures and test a restore!
Yes. A full snapshot is going to contain addon data. If you have a 20GB database you are going to end up with large snapshots.
It would also be simple to try a snapshot and then check it for yourself. A little experimentation and curiosity can go a long way. Waiting for an answer to a post from 2019 is less productive.