I don't really understand how to use this to build a real backend. Everything seem to be revolving around reactions (new thing happening in the DB, file uploaded etc) but isn't the more common use case that you actually want to call the function directly?

I'm building a simple React Native/Expo app where I want users to join random chat rooms so I obviously need some logic to delegate this process. Is Firebase/Cloud Functions something I can consider?

My first thought was "oh, maybe I can let my app call the function directly to add the user to a random chat room" but I don't see how that is possible.