Unity　全シーン管理マネージャーの作成 ref: https://qiita.com/tsuzukihashi/items/1d8a9fce4465c688c78a

DDManager.cs

using UnityEngine;

public class DDManager : MonoBehaviour {

    public GameObject gameObject;

    void Start () {
        DontDestroyOnLoad(gameObject);
    }

}

ManagerSceneAutoLoader.cs

using UnityEngine;
using UnityEngine.SceneManagement;

public class ManagerSceneAutoLoader : MonoBehaviour {

    [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.BeforeSceneLoad)]
    private static void LoadManagerScene(){
        string managerSceneName = "ManagerScene";

        //ManagerSceneが有効でないときに追加ロード
        if(!SceneManager.GetSceneByName(managerSceneName).IsValid()){

           SceneManager.LoadScene(managerSceneName, LoadSceneMode.Additive);

        }
    }
}