What happens if I build 2 scenes in unity code example

Example: 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.