That depends entirely on the hotel. They can use one of the many booking systems that integrate with booking.com through XML or they can choose not to. If they choose not to, then they will have to manually sync availability and receive their bookings by e-mail. A nightmare, but their own fault for not running their business properly.

> A friend of mine recently had to sleep in the car because booking never confirmed her reservation with the hotel and wouldn't accept her booking.com confirmation printout.

I'm 100% sure this was the hotels fault.