The older ones used a linear potentiometer. The newer ones eliminated the encoder and used a stepping motor. Coarse focus was achieved by automatically calibrating the focus lense position to the zoom setting, zoom also done with a stepping motor. Both stepping motors initialized by moving to a home location when power is first applied to the camera and then keeping track of the step count. Fine focus is achieved by moving the focus lense back and forth slightly and analyzing the frequencies present in the video. The idea being that the maximum amplitudes of high frequencies will be greatest at the best focus.
The automatic calibration routine was built into the lense control microprocessor. When enabled, the routine steps through many zoom positions with the camera viewing a special chart. The micro remembers the best focus position for each zoom setting and has an interprolation routine for positions in between the zoom steps utilized in the procedure.