how do I check-out the guests? Do I need to do it or the guest does it himself?

[Sandra126]

@Armando8, You can do this how you wish. I tell guests to leave keys inside and close the door which locks automatically. If you live there, you might want to say goodbye and thanks in person!

You don't have to do anything Airwise, it is assumed that guests leave as planned.

[Ana7]

The system assumes the guest left on the day his booking ends. You don't have to do anything special about that.