Why won't my main menu be my start screen despite being set as start screen?

How do I...?
1

I tried making a main menu, the menu that you start at. I already marked it as a start scene, but why won’t that scene be the start scene and instead use the gameplay scene?

2

Is this in preview mode, or once it’s compiled and released?

If it’s once it’s compiled, then what are the events of the main menu mode?