There are two types of networked game objects in Mirror’s multiplayer system:
Those that are created dynamically at runtime
Those that are saved as part of a Scene
Game objects that are created dynamically at runtime use the multiplayer Spawning system, and the prefabs they are instantiated from must be registered in the Network Manager’s list of networked game object prefabs.
However, networked game objects that you save as part of a Scene (and therefore already exist in the Scene when it is loaded) are handled differently. These game objects are loaded as part of the Scene on both the client and server, and exist at runtime before any spawn messages are sent by the multiplayer system.
When the Scene is loaded, all networked game objects in the Scene are disabled on both the client and the server. Then, when the Scene is fully loaded, the Network Manager automatically processes the Scene’s networked game objects, registering them all (and therefore causing them to be synchronized across clients), and enabling them, as if they were spawned at runtime. Networked game objects will not be enabled until a client has requested a Player object.
Saving networked game objects in your Scene (rather than dynamically spawning them after the scene has loaded) has some benefits:
They are loaded with the level, so there will be no pause at runtime.
They can have specific modifications that differ from prefabs
Other game object instances in the Scene can reference them, which can avoid you having to use code to finding the game objects and make references to them up at runtime.
When the Network Manager spawns the networked Scene game objects, those game objects behave like dynamically spawned game objects. Mirror sends them updates and ClientRPC calls.
If a Scene game object is destroyed on the server before a client joins the game, then it is never enabled on new clients that join.
When a client connects, the client is sent an ObjectSpawnScene spawn message for each of the Scene game objects that exist on the server, that are visible to that client. This message causes the game object on the client to be enabled, and has the latest state of that game object from the server in it. This means that only game objects that are visible to the client, and not destroyed on the server, are activated on the client. Like regular non-Scene game objects, these Scene game objects are started with the latest state when the client joins the game.