Had a play with some blend motors, as you do
I had a set come with the car, thought I'd have a look inside them to see what's there.
A small motor with an initial worm drive through a very large reduction gear chain to the keyed output shaft.
A potentiometer geared to the output shaft sends an analogue voltage to the HEVAC controller which is proportional to the flap position. The two gears have 'timing marks' in the form of arrows so that the relationship between the flap position and the voltage feedback is maintained.
Not a lot to go wrong in there.
Peter