I’m guessing this topic arose from OP’s interest in this topic:
where it was discussed how a needle valve could be operated to control the cooling water flow in a distillation column.
That discussion prompted the interest in this topic:
where the use of a multi turn servo was proposed to operate the needle valve.
OP then asked how servos were controlled, hence this topic.
The distillation topic OP suggested that controlling a water pump via PWM and as the discussion progressed it was suggested that perhaps controlling a valve might be a better approach. The topic is ongoing.
The topic that spawned this topic centered around using a multi turn servo to operate a needle valve. That topic is also ongoing.
My thought is: if the needle valve can be operated in its middle range and it is not necessary for it to be full on or full off, then a multi turn servo with the torque available to turn the valve stem might be the solution. If however it is necessary to completely close or open the valve to get the flow control needed then another approach that included a way to sense the stalled positions at the extremes would be required.