unity multiple scenes open code example

Example 1: unity how to load up a scene

SceneManager.LoadScene("scene_01");

Example 2: unity load multiple scenes

It is recommended to avoid using DontDestroyOnLoad to persist manager 
GameObjects that you want to survive across scene loads. Instead, 
create a manager scene that has all your managers and use 
SceneManager.LoadScene(<path>, LoadSceneMode.Additive)
and SceneManager.UnloadScene to manage your game progress.