Scenes contain the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces.
When you create a new Unity project, your scene view displays a new Scene. This Scene is untitled and unsaved. The Scene is empty except for a Camera (called Main Camera) and a Light (called Directional Light).
To save the Scene you’re currently working on, choose File > Save Scene from the menu, or press Ctrl + S (Windows) or Cmd + S (macOS).
Unity saves Scenes as Assets in your project’s Assets folder. This means they appear in the Project window, with the rest of your Assets.
To open a Scene in Unity, double-click the Scene Asset in the Project window. You must open a Scene in Unity to work on it.
If your current Scene contains unsaved changes, Unity asks you whether you want to save or discard the changes.
It is possible to have multiple Scenes open for editing at one time. For more information about this, see documentation on Multi-Scene editing.