I'm an enthusiast and have only flown flight/atc simulators (on the VATSIM network). Obviously sometimes the airspace you are flying in is not covered by any ATC (so I have to set my frequency to Unicast). And very often there is only the tower of an airport available. How is it in the real world?
Is the whole world's airspace covered by ATC at all times?
Is any airport covered by all its layers of ATC (Delivery, Ground, Tower, ...) at all times?
How often do pilots use the Unicom frequency because there is no ATC available in current airspace?