Scene Loading Question

I want to discuss general question related to scene loading and unloading.

For example, I have three scene in my game say Main Menu, Game Play and Game Over Scene.
Then at initial loading time how many scene get loaded into internal memory and graphical memory of device?

Or is there inbuilt strategy that at a time single active scene get loaded into device memory?

If you use LoadLevel then only one scene is loaded at a time. You can merge scenes by using LoadLevelAdditive and you can have some objects remain between scene loads using DontDestroyOnLoad