A simple example for "How to build a multiplayer game with Mirror" is Pong. It illustrates the usage of `NetworkManager`, `NetworkManagerHUD`, NetworkBehaviour, NetworkIdentity, `NetworkTransform`, `NetworkStartPosition`and various Attributes.
The Additive Scenes example demonstrates a server additively loading a sub-scene into a main scene at startup, and having a server-only trigger that generates a message to any client whose player enters the trigger zone to also load the sub-scene, and subsequently unload it when they leave the trigger zone. Only players inside the trigger zone can see the objects in the sub-scene. Network Proximity Checker components are key to making this scenario work.
The Lobby System example demonstrates how to set up a "staging" scene where players assemble before starting a match. When all players are ready, the server sends them all a message to change scenes (along with the server itself) to the actual game play scene so they all come in at once.