I have managed to basically connect with my friend over internet, by forwarding my IP address from my router settings... this is not viable because there are few people willing to do what I did to play games with their friends. So how to actually do UDP hole punching (basically what I did manually to my router) in unity using the mirror networking solution...
You can use Photon Bolt or Photon Fusion to let players host a game on their local machine like minecraft, etc. Photon provides relay as well as tries to use STUN to establish direct peer to peer connection to the host via UDP. PUN2 is also a good choice, although I like Photon Bolt/Fusion better - it's less of a simple RPC framework and more programmer oriented. Also, PUN does not do any STUN direct peer connection, it will always be relayed. Photon Bolt and Fusion will first attempt a STUN direct peer connection and then fallback to relay if necessary. It's been around for years and is the best choice.
Sure you can develop a Unity game with Mirror (although without a relay built in) but it's not nearly as easy to setup and use as Photon Bolt/Fusion and they don't provide a relay. As someone mentioned, you might be able to hack something together in some way but yeah - not recommended.
Ugh, yeah don't use WebRTC for a Unity game (or probably anything other than streaming music/video like it was made for to be honest).
Unity's MLAPI is "under development" and their last API was suddenly dropped "deprecated", so I wouldn't use that.