Main.cs

using System;
using Godot;

public partial class Main : Node3D
{
    [Export]
    public PackedScene PlayerScene;

    [Export]
    public PackedScene RemotePlayerScene;

    MultiplayerSpawner _spawner;
    Node3D _spawn;

    public override void _Ready()
    {
        _spawner = GetTree().CurrentScene.GetNode<MultiplayerSpawner>("MultiplayerSpawner");
        _spawner.SpawnFunction = Callable.From<int, GodotObject>(CustomSpawn);

        _spawn = GetTree().CurrentScene.GetNode<Node3D>("spawn");

        if (!Multiplayer.IsServer())
            return;

        Multiplayer.PeerConnected += SpawnPlayer;
        SpawnPlayer(1);
    }

    void SpawnPlayer(long id)
    {
        _spawner.Spawn((Variant)id);
    }

    Node CustomSpawn(int id)
    {
        Node3D player;

        if (id == Multiplayer.GetUniqueId())
        {
            player = PlayerScene.Instantiate<Node3D>();
            GD.Print("My id: ", id);
        }
        else
        {
            player = RemotePlayerScene.Instantiate<Node3D>();
            GD.Print("Remote id: ", id, " My id: ", Multiplayer.GetUniqueId());
        }

        player.SetMultiplayerAuthority(id);
        player.Name = id.ToString();
        return player;
    }
}