I came across this video of a man showing the components in a hydraulic fan drive used on a school bus. He also explains some of the operation.
However, he repeatedly calls the hydraulic motor (which drives the fan) a pump. It's not a pump; it's a motor — in this case a gear motor. The hydraulic pump (the vehicle's power steering pump, in this case) transmits rotational power from the engine as hydraulic power to drive the motor. The hydraulic motor, in turn, transmits the hydraulic power as rotational power to drive the fan.
Otherwise, I think this is a good video — easy to see and hear everything.