EntryBehaviourBase.cs

using Cysharp.Threading.Tasks;
using UnityEngine;
using UnityEngine.SceneManagement;

namespace Nekomimi.Daimao.SceneEntry
{
    public abstract class EntryBehaviourBase : MonoBehaviour
    {
        public Scene Scene => gameObject.scene;

        public abstract UniTask PreEntry(EntryEntityBase entryEntity);

        public abstract void Entry();

        public abstract UniTask PreLeave();

        public abstract UniTask PreUnload();
    }

    public abstract class EntryEntityBase
    {
        public abstract string SceneName { get; }
    }
}

SceneEntryLoader.cs

using System.Collections.Generic;
using System.Linq;
using Cysharp.Threading.Tasks;
using UnityEngine;
using UnityEngine.SceneManagement;

namespace Nekomimi.Daimao.SceneEntry
{
    public static class SceneEntryLoader
    {
        private const string LoadingSceneName = "LoadingScene";

        public static async UniTask EntryScene(
            EntryEntityBase nextEntity,
            EntryEntityBase[] additive = null)
        {
            await UniTask.Yield();

            var currentScenes = GetAllLoadedScene();

            await SceneManager.LoadSceneAsync(LoadingSceneName, LoadSceneMode.Additive);
            var loadingScene = SceneManager.GetSceneByName(LoadingSceneName);
            SceneManager.SetActiveScene(loadingScene);

            var currentEntry = currentScenes
                .Select(DetectEntry)
                .Where(entry => entry != null)
                .ToArray();

            foreach (var entry in currentEntry)
            {
                await entry.PreLeave();
            }

            var nextSceneEntities = new List<EntryEntityBase>();
            if (additive != null)
            {
                nextSceneEntities.AddRange(additive);
            }

            nextSceneEntities.Add(nextEntity);

            var nextSceneName = nextSceneEntities
                .Select(entity => entity.SceneName)
                .Distinct()
                .ToList();

            foreach (var entry in currentEntry)
            {
                if (nextSceneName.Contains(entry.Scene.name))
                {
                    continue;
                }

                await entry.PreUnload();
                await SceneManager.UnloadSceneAsync(entry.Scene);
            }

            await UniTask.Yield();
            await Resources.UnloadUnusedAssets();
            await UniTask.Yield();

            var nextEntry = new List<EntryBehaviourBase>();

            foreach (var entity in nextSceneEntities)
            {
                var entry = await LoadSceneAndEntryAsync(entity);
                if (entry == null)
                {
                    continue;
                }

                await entry.PreEntry(entity);
                nextEntry.Add(entry);
            }

            var nextScene = SceneManager.GetSceneByName(nextEntity.SceneName);
            SceneManager.SetActiveScene(nextScene);
            await SceneManager.UnloadSceneAsync(LoadingSceneName);
            await UniTask.Yield();
            foreach (var entryBehaviourBase in nextEntry)
            {
                entryBehaviourBase.Entry();
            }
        }

        public static Scene[] GetAllLoadedScene()
        {
            return Enumerable.Range(0, SceneManager.sceneCount)
                .Select(SceneManager.GetSceneAt)
                .ToArray();
        }

        private static async UniTask<EntryBehaviourBase> LoadSceneAndEntryAsync(EntryEntityBase entity)
        {
            if (!SceneManager.GetSceneByName(entity.SceneName).isLoaded)
            {
                await SceneManager.LoadSceneAsync(entity.SceneName, LoadSceneMode.Additive);
            }

            await UniTask.Yield();
            var scene = SceneManager.GetSceneByName(entity.SceneName);
            return DetectEntry(scene);
        }

        private static EntryBehaviourBase DetectEntry(Scene scene)
        {
            if (!scene.isLoaded)
            {
                return null;
            }

            return scene.GetRootGameObjects()
                .Select(rootGameObject => rootGameObject.GetComponent<EntryBehaviourBase>())
                .FirstOrDefault(entry => entry != null);
        }
    }
}