Mirror uses GUID for Asset Ids. Every prefab with a NetworkIdentity component has an Asset Id, which is simply Unity's AssetDatabase.AssetPathToGUID converted to 16 bytes. Mirror needs that to know which prefabs to spawn.
Mirror uses uint for Scene Ids. Every game object with a NetworkIdentity in the scene (hierarchy) is assigned a scene id in OnPostProcessScene. Mirror needs that to distinguish scene objects from each other, because Unity has no unique id for different game objects in the scene.
Mirror uses uint for NetId. Every NetworkIdentity is assigned a NetId in NetworkIdentity.OnStartServer, or after spawning it. Mirror uses the id when passing messages between client and server to tell which object is the recipient of the message.
Every network connection has a connection id, which is assigned by the low level Transport layer. Connection id 0 is reserved for the local connection when the server is also a client (host)