When you click Play on your Godot project, it will automatically launch on your default monitor. For my setup, that’s usually not convenient. Fortunately you can change where the game appears in the Editor settings.

Editor -> Editor Settings -> Run -> Window Placement

You can have the game launch on the monitor of your choice, and even in a specific position if you like. I usually have it launch in the middle of my screen on the same monitor as my editor.