It is done in the outlet valve on the pump. On this one the engineers did their homework.
The valve senses the pressure after the pump, when the steering is idle they run a set pressure from 80 to 150 psi and maintain that. Recirculating ball and rack systems will be at either extreme there. If you ever get to look at the schematic for a hydraulic fuse it may help make sense how this is done.
This low pressure is what controls the on center feel of the steering. From this setting the pressure can ramp up to the peak pressure to help steer the vehicle. Most all of this is done in this valve on these steering systems.
The American designed systems do much of the flow control in the steering box, this being part of the reason American PS pumps do not work on the Japanese steering units.
What this means to a driver, when driving with the American style steering there is quite a difference in steering effort with the engine at idle as opposed to revving at 2000 rpm. In many of the Japanese vehicles there is little change.