Can you use it as a standalone messaging app or something? Like telegram? 🤔

Yes. It is a dedicated messaging client.

Whats it doing diff than keychat?

That said, Nip 17 does do some things that NIP-EE seems not able or not ready to do. This from NIP-EE: "It is not possible to share group state across multiple Clients. If a user joins a group from 2 separate devices, their state is separate and they will be tracked as 2 separate members of the group." That's pretty huge, since we all live cross-device these days. It's not a limitation of MLS, but it could be a limitation forced by how MLS integrates with Nostr in this particular way, I'm not sure. Either way, on NIP 17 as a single npub you can operate cross-device (say Macbook and iPhone) with relative ease.

NIP-17 won't die, it's better suited for social media app capabilities, and it's enough secure for that usecase too, it just sucks for groups. NIP-EE is good for the signal usecase

And what is the difference between keychat and 0xchat?