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They sense rpm. All bl controlers know rpm at all times. This is sensed as "back emf" via the motor leads. Or more accurately, they know which magnet pole is over stator pole for "commutation ".
Gov mode truly tries to hold a set rpm. I think the fact many fbl controllers have gov function has confused you. In that instance the rpm feedback for the gov control loop must come from an external sensor. For a natively governed esc, rpm sensing and the rpm control loop are built in.