Does it choose based on the surface, or just let both run all the time? (Mop on carpet doesn't sound ideal)

https://www.youtube.com/watch?v=Tz6U9QkXn5I Check this for a short overview of the available robovac mop technologies - modern roller mop equipped units have ultrasonic carpet sensors and will actually cover the entire mop to avoid getting carpets wet.

They have an ultrasonic sensor to measure the type of floor and will not pump water when carpet is detected