I did some research about this guy's atat a while back (so you just have to take my word for it unless you want to find the information yourself) -- but the model is built from scratch, though I'm not sure if he did both the model himself and the mechanics, or had someone else do the model. He also was pretty dead set on not revealing how the mechanics work.
Here's a screen grab from a video of his model. You can see with the back leg, there is a wire going into the top of the leg. My guess is that it uses a chip to run different motors. It just doesn't seem like he did the typical gears and such powered by one motor.
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