Welcome! A scene is just a snapshot of the specified devices at a moment in time. So say hallway light on at 50%, living room light on at 100%, porch light on at 100%, door unlocked, might be a coming home scene. But you can’t use it to send notifications. And it doesn’t in and of itself get triggered from anything.
An automation is an if/then rule. One of the “thens“ you can do is activate a scene. So you might have “if my phone is detected as arriving home, then activate the coming home scene.”
In your case, because you want to use the motion sensor as an “if“ then that’s an automation. And you can also have the automation send a notification.
I can’t say anymore about specific details because I’m not up-to-date on the current app, but I’m sure someone else will be able to help.
p.s. The purpose of defining a scene is so that when you have several devices which you commonly change all at the same time, but maybe to different settings, you can put those in a scene and then activate it from an automation. It just keeps you from having to add each individual device each time when you want to use those same settings. Also, you used to be able to use scenes as widgets, but I don’t know if that still possible. I think you can also activate scenes through the voice assistants, but again I’m not up-to-date on the details of the current app.