Check out example codes for "custom player spawner mirror". It will help you in understanding the concepts better.

Code Example 1

//You need to create a message handler
public class MMONetworkManager : NetworkManager
{
    public override void OnStartServer()
    {
        base.OnStartServer();

        NetworkServer.RegisterHandler<CreateMMOCharacterMessage>(OnCreateCharacter);
    }

    public override void OnClientConnect(NetworkConnection conn)
    {
        base.OnClientConnect(conn);

        // you can send the message here, or wherever else you want
        CreateMMOCharacterMessage characterMessage = new CreateMMOCharacterMessage
        {
            race = Race.Elvish,
            name = "Joe Gaba Gaba",
            hairColor = Color.red,
            eyeColor = Color.green
        };

        conn.Send(characterMessage);
    }

    void OnCreateCharacter(NetworkConnection conn, CreateMMOCharacterMessage message)
    {
        // playerPrefab is the one assigned in the inspector in Network
        // Manager but you can use different prefabs per race for example
        GameObject gameobject = Instantiate(playerPrefab);

        // Apply data from the message however appropriate for your game
        // Typically Player would be a component you write with syncvars or properties
        Player player = gameobject.GetComponent<Player>();
        player.hairColor = message.hairColor;
        player.eyeColor = message.eyeColor;
        player.name = message.name;
        player.race = message.race;

        // call this to use this gameobject as the primary controller
        NetworkServer.AddPlayerForConnection(conn, gameobject);
    }
}

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