A shared project is nothing monogame-specific.
It’s a template in visual studio.
The way I usually setup cross-platform monogame-projects is somewhat like this:
MyGameSolution
- MyGame (Shared Project, this is where your game1.cs will go)
- MyGame.DesktopGL
- MyGame.Android
- MyGame…
The Shared Project is then referenced from the Platform-Projects.