Writing wise, it is similar to discord, with a capability for full end-2-end encryption of the contents. Because of how messages are stored, it tends to be slow to go through the backlog (unfortunately).

You can do media embedding, videos, pictures, source code, etc. Because of how history works (and depending on encryption settings) searching has always felt weak to me. Discord search capabilities are quite amazing in comparison.

For finding rooms and communities, you can definitely search for it by name and join the different rooms. You will be searching in a specific server. The interesting piece is you can create a room in server A, and create an alias for it on server B (you must have a local account on server B, the alias can be the same name or different). This makes anyone joining either room go to the same place.

Outside that, you will find several open-source projects either having a room on matrix.org or hosting their own small server (with maybe an alias somewhere else). This just mimics how things used to be on IRC channels. There's an IRC integration that merges the communications between an IRC channel and a matrix channel. I think there's also one for discord, so if you type on matrix, the bot types on discord (and vice-versa, but no audio).

Easiest thing to do, go to matrix.org, create an account, explore. You can always delete it or create an account somewhere else later.

If the room is encrypted, not even the server is able to read your messages, only people in that channel, which can also be just 2 people/direct messages.