I just signed up and tried to figure out how to join any remotely interesting community. There were none.
There were a bunch of public communities - not many - and I think the most interesting ones in the public room list were either Element Android, Telegram, and Rust. And Rust is one of my most hated games that I wish I'd refunded.
They created the "inbox" button on the desktop client because when you're in a ton of servers, finding where the notification was a big pain point. The notifications tab covers the same thing for the app, and it's very readily accessible now. I think before the update it still took about 3 taps to get to the "Inbox", and only then if you knew where it was hiding.