Engine cooling is done via the heat exchanger (radiator) by passing as much air through the vents on the heat exchanger as possible. The radiator fan is on a clutch (engine driven) or on a thermostatic control unit (electric fans) so as to vary the speed of the fan, slowing the fan down when the ambient airflow is colder and speeding it up when the airflow is hotter. But in either case, it's the air flowing through the radiator which causes the exchange of heat.