hope i can help a little, although will keep it very brief as the complexities of a modern airliners control system cannot be fully explained here.
firstly as you have already been told, any input by either pilot will disconnect the autopilot (that is to say a force applied in excess of a predetermined figure).
next, the three primary control surfaces, (rudder, elevators and ailerons) are controlled by a cable system linked to the control column ( or rudder peddals) which utilise cable quadrants,bellcranks and push rods to operate a servo valve on the assossiated power control actuator. movement of the servo valve allows hydraulic pressure to drive the body of the actuator, which is connected to the control surface, to a position proportionate to the control column displacement. The cable runs are duplicated and the control surface pca's are triplicated. (757).
This is a very very basic overveiw, as the system although now old technology is far more complicated than can be fully explained here. once you start adding autopilots, temp compensation, airspeed factors etc, it cannot be explained to its full outside of a classroom on a full type coarse.
hope that helps.