They build and program many things by themselves. Recently, they completed a very impressive addition: an airport, complete with starting and landing airplanes, cars etc.
I listened to an interview with one of the founders, Gerrit Braun, where he says that for the airport alone they wrote several hundred thousand lines of code.
Camera mounted on train:
Now if only I'd made it before MIT destroyed building 20.
You can see the tetris in action