Does it automatically updates the changelog? Or it must be done manually?

Also having some docs for contributors would help. For example, it looks like if you would name this branch maintenance-something, it would get autolabeled with maintenance tag and release drafter would do a better job with the changelog generation...

Currently we don't display that information anywhere. With the commitlint we at least had a github check even though most people just ignored it :)

I want to inject (separate PR) changelog updating, as well. I view these all as separate PRs, since I like to slice.

After this merges, future releases (from mainlines) will have auto-generated release notes... There is some way to handle this multi-branch.. I haven't done it yet.