When you press the start button, it goes through everything and vacuums. As the thresholds between the rooms are so high that the robot can't get over them, we vacuum room by room. We do it exactly the same way, place the robot in the room, close all the doors and press the button. It then vacuums the room. We never need the app. But we don't use the wet pick-up function. I'm no longer sure whether you need the app for that.

I would definitely use the app because it is the only way to set the different suction levels and damp mopping. The S12 also creates a map of the home, which you can use later - also via the app - to have individual rooms cleaned.
None of this works without the app.
Unfortunately, you have to register with Xiaomi, but they also accept @mozmail addresses, which makes it OK again.