Climbers walk off the back. The face of El Cap is the hard part, the back is just a series of rocky hills that lead down to the floor of the valley. The hike down takes a few hours and hiking just to look out across the valley is a popular activity on it's own. You can rappel back down as well but you either need a bunch of fixed rope or to deal with constantly rethreading the rope and swinging to the belay station every pitch or two. To deal with this many big wall routes have fixed static ropes set up for jugging up and rappeling down. This is how the people filming access the wall.