Basically, they have a system of tanks, mixing vessels, valves, and pumps that move a bunch of caustic solution (PBW is one example) through all the lines. The benefit is that you aren't moving equipment; neither the cleaning skid nor the production skid moves anywhere. Hence, clean-in-place.
There's also usually an SIP system, which is sanitize/sterilize-in-place. Same idea, just with sanitizer and/or steam.
These are common food and pharma industry processes.