Wikia

The Nexus is a hub to almost all the other locations. Young has the ability to Return to Nexus from almost every outdoor location in the game. The Nexus contains a series of portals to 21 different locations. These portals are activated as Young finds their destinations throughout the land.

Bright gems glow at the top of each door when all the standard cards in that area have been found. No indication is given regarding post game content cards or secrets.