The Network Match Checker component controls visibility of networked objects based on match id.
Any object with this component on it will only be visible to other objects in the same match.
This would be used to isolate players to their respective matches within a single game server instance.
When you create a match, generate and store, in a List for example, a new match id with System.Guid.NewGuid(); and assign the same match id to the Network Scene Checker via GetComponent<NetworkMatchChecker>().matchId.
Mirror's built-in Observers system will isolate SyncVar's and ClientRpc's on networked objects to only send updates to clients with the same match id.