Creating link between Firebase real-time database and authenticated users

So far I have successfully implemented Firebase within my Android application, where I can add users to the Authentication portal through a SignUpActivity, and also add maintenance issues to the real-time database through a MaintenanceActivity.

However, at present, none of the database data is linked to specific users, which is what I want to achieve. So essentially at the moment when I log in as an arbitrary user, the same data will always come up.

Presumably, and having read several other threads on this, the User UID will be required here and will need to be present for every maintenance record.

I’m not sure, however, how I can implement this. Possibly a layer of authentication needs implemented into the MainActivity?

Finding it hard to get my head around this, so any help on this would be much appreciated.