You can also use @nextcloud to set up your own private messaging service:
https://growyourown.services/review-of-talk-app-for-nextcloud
The service has its own dedicated mobile app for Android and iOS too.
Major downside is it doesn't have any kind of federation (yet), so you can only talk to other people on your server.
If this sounds like something you'd want (for example an internal chat system for your friends, family or organisation), it might be worth looking at this.
No, it's not XMPP.
There is an XMPP add-on for NextCloud, but it has no connection to the NextCloud Talk add-on.
It would be nice if Talk used XMPP but unfortunately it doesn't.