Hello @Minahora - this is a quite “complex” setup including the installation and setup of several services. I remember having tried this (XMPP server with NC) almost some 2 years ago, and it took me quite a time to bring it up. I then had to stop my attempt, because I didn’t have enough experience and ressources for setting up also a turn server, which is needed for video/audio being behind NAT. I only managed to setup a local TURN server some days ago for NC Talk -> TURN server for Nextcloud Talk
So it contains several issues - don’t be too frustrated, if nobody can deliver a quick all-in-one recipe, it’s about doing a lot of research, trial’n error 
- Setup XMPP server
- Setup NC server
- Link both
- Need for STUN/TURN server? User external or setup these yourself
regards