There is an off-ramp at every gate in the United Airlines terminal. The bags make their way down a short conveyor to a sorting station on the ground at the gate.
At the sorting station, baggage handlers load the bags onto carts or into special containers that go right into the airplane. When loading the plane, bags that will be making a transfer after the flight are loaded into separate areas than bags that will be heading to baggage claim. A monitor at the sorting station tells the handlers which bags are going where (remember, the baggage-handling system always knows exactly where each bag is going).
After the bags are loaded into carts or containers, they are brought the short distance to the plane and loaded. Some planes are bulk loaded, meaning the bags are brought up one-by-one on a conveyor and placed into shelves in the cargo hold. Other planes are container loaded, meaning that special containers are loaded on the ground and then placed into the plane.