How to connect to a P2P camera

I bought an IP camera with P2P capabilities and I am struggling to understand how it works. There is an ID at the bottom of my camera that I put on the android app and it just works, no matter if it's behind the router. Now I am trying to reproduce this using coding, but I have no idea where to start. I found this Android IP camera streaming app using P2P mode over mobile port? This is what I am trying to do, but still I am lost on how it works. Any ideas?