The main components of a servo motor includes a High Speed Low Torque DC Motor, a Reduction Gear Box, Potentiometer and a H-bridge Circuit Board.
The DC motor is coupled to a reduction gear box to reduce the rpm of the output shaft and increase torque.
The output shaft is coupled to the potentiometer and rotates along with it.
When the motor rotates the potentiometer, sending voltage as signal and this voltage is relative to the angle of the output shaft.
The input signal is compared with potentiometer signal to get the desired Angle to the output servo arm.
It activates an H Bridge circuit that enables the motor to run in both clockwise and counter clockwise direction until the signal difference from the potentiometer and input is zero.
These servos usually can spin up to 180 degrees depending on the potentiometer output.