Two kinds of multiplayer, and which one your game needs

Set a project to Co-op or Versus and players who open your link see each other, see each other move, and share whatever your rules decide. Nothing to author. That is the relay model, and it suits co-op, exploration and turn-based games well.

What the relay does not do

The relay forwards what players send. It does not simulate movement or check hits, so it is not a referee. One player runs the rules on behalf of the room and hands over if they leave. For friends playing together, that is exactly enough.

What Sunder does instead

Sunder is a competitive shooter, so it runs a dedicated server that simulates the world and answers with the truth. Prediction keeps movement instant and reconciliation quietly corrects disagreements. What you see is what the server saw.

That server tier exists on the platform today for Sunder. Bringing it to every project is a larger piece of work than presence and shared state, and it is the direction for competitive projects.