Example of an autoload scene manager which loads a common scene & loads scenes as children of a viewport in that common scene

SceneManager.gd

extends Node

const COMMON_SCENE = preload("res://content/scenes/sceneroot.tscn");
const DEFAULT_SCENE = preload("res://content/scenes/godot_splash.tscn");

var _sceneroot: Node;
var _loadedScenes: Array[Node] = [];

func _ready() -> void:
    # instantiate common scene root
    var common = COMMON_SCENE.instantiate();
    add_child(common);

    # get main viewport (loaded scenes should be added as children of this viewport)
    _sceneroot = common.get_node("main");

    # load default scene
    load_scene(DEFAULT_SCENE);
    pass

# load a new scene, unloading previously loaded scene(s)
func load_scene(scene: PackedScene) -> Node:
    for loadedScene in _loadedScenes:
        loadedScene.queue_free();
    _loadedScenes.clear();
    
    var instance = scene.instantiate();
    _sceneroot.add_child(instance);

    _loadedScenes.append(instance);
    return instance;

# load a new scene, adding it to existing loaded scene(s)
func load_scene_additive(scene: PackedScene) -> Node:
    var instance = scene.instantiate();
    _sceneroot.add_child(instance);
    _loadedScenes.append(instance);
    return instance;