Most cities get their water supply from local sources (springs, rivers, lakes) so the pressure in the system is never more than what is produced by the pumps.
But some cities get their water from elsewhere, typically from a much higher elevation.
Portland Oregon for instance gets its water from Bull Run Watershed, which is more than a kilometre higher than the city.
With that elevation difference, if there is no significant flow, the static water pressure in the city pipes would be over 100 atmospheres, which clearly isn't possible.
How is water pressure prevented from becoming catastrophically high?