I'd bet it refers not to the capabilities of the API per se, but the fact that you can integrate the API in your own user management system (if you had one already) or build one specific for you and your team.
Does it mean that i can add custom fields to the user objects, if yes how ?
I haven't used it, but isn't this more involved in customising the user?:
- Update user properties - including their password - without having to sign in as the user.
Could you add these extra fields in a different document and link the users via the uuid? I guess keeping these documents clean (especially these one, being fairly critical to be fast and reliable) is way better than stuffing them with plenty of data.
One last thought about your question. If Firebase allowed anyone to modify these fields arbitrarily from the Realtime Database, we wouldn't have an Admin library to do so, would we?