Our flight back home required an airplane change at Dulles International. The gates used the smaller regional aircraft often have airplanes to or from several locations. That means as you leave for your next destination, right next to you is another airplane heading some place else, and the passengers went through the same door you did. The staff is very careful and check tickets are each point of possible confusion to be sure everyone gets on the proper plane.
One nice, and rather fun, thing is that you are right close to the ground as you see here. It is as if you are in the midst of the action, which you are, in a sense.