How can I save a three different scenes in a one project?

There is a three scenes in the following video tutorial

Youtube Link

How can I save a three different scenes in a one project?

For saving:

Go to File>Save Scene or press Ctrl+S in Windows and Linux, and Cmd+S in MacOS

For creating a new Scene:

Go to File>New Scene or press Ctrl+N in Windows and Linux, and Cmd+N in MacOS

To save a scene, press Ctrl+S on windows and command+S on Mac.
Now, you can find “Add Open Scene” by selecting File > Build Setting to include the scenes in the final build. You can drag & drop all those scenes in the “Scenes In Build” column, which you want in the final build. Make sure to keep them checked. But, the game starts with only 1 scene and then you have to call the other scene with some event using SceneManager.LoadScene(). You can call any scene from the “scenes in build” from any script.