itch.io

The main idea is that for 2 connected portals you have 2 offset cameras(so 6 in total for our game), that follow the players position and project what the cameras see onto a plane that is in the middle of a portal model(a shader helps with a correct projection). Then you have 2 connected colliders via a script that teleports you to the other portal. (you can google brackeys portals for more details)