With the exception of the newest vehicle designs that use electric power steering, most power-assisted steering systems use a hydraulic pump and valves.
source/image(PrtSc): Gary’s Tradition Auto
The pump is turned continuously when the engine is turning, generating high pressures in the power steering fluid. When the steering wheel is turned, a valve in the steering rack and pinion assembly directs the high-pressure fluid into one of the chambers inside the rack and pinion assembly. Watch the video from Gary’s Tradition Auto for more info:
This pressurized fluid helps to push the steering rack in the direction the driver is turning. Over time, contaminants enter the power steering system and collect in the power steering fluid. These contaminants can cause wear on the rotary vane pumps.
Worn Pumps can be noisy, have inadequate pressure, or develop leaks. Power Steering fluid should be replaced periodically to ensure the long life of the seals inside the power steering system.//Gary’s Tradition Auto