I don't think he found any clay, or at least not much of it. This is the tutorial world (we know this because the village on the far left and castle on the far right are what exist in the tutorial world). There is a brick gazeebo structure in the tutorial
world. You can actually see in the first image that the brick was pulled off of the gazeebo. So what he did is take the bricks from the tutorial world structure and duplicated the bricks using the duplication glitch. The duplication glitch is also reflected
in the train tracks, which are all powered rails, which provides no benefit to minecart travel (one powered rail combined with 20 standard rail provides equally fast travel) and would only be used if gold is duplicated. Certainly not judging--just noting
that there is not enough clay is any game world at the moment to support the amount of bricks used in the images. This was done in the Xbox 360 Creative Mode (aka the Duplication Glitch).
You have a lot going on in your world. It looks really good. I really like the subtle details such as the small waterfall flowing down the hillside. I also noticed you managed to find some clay on your map, something I've yet to find myself on my main world.
Way to go! Nice use of the rail as well, as a fast travel across the waterway.