I Have added a Main Menu with Buttons to Start the Game and To Quit.
To Start The Game I use Unity Scene Management API and Load The Scene.
To Quit the Game I call the Static Game Manager Quit Method.
The Quit Method use preprocessor directives since Application Quit does not work on all Platforms. I am building for Android but I am also testing in the Editor and I will be building a WebGL for use on my portfolio site so the game can be played without the android device. To quit my game on an Android Device I did a google search for “application.quit unity android” which gave me https://stackoverflow.com/questions/45636512/application-quit-wont-work-on-android. The Android Java Object is a Class in Unity. I have to call the Static Method current Activity Which I can Call the finish Method, there are plenty of examples of how to do things in Android from Unity on the web.