Can someone please explain Chongqing to me?

here

I am having a hard time understanding how it is layered like this. How did they build this? This seems out of this world.

I am not sure how much more details to add. The link is in the post. However, this city has multiple levels to it, and thousands of stairs. I think it is built between mountains. What I don’t get, is how they built the city, and why it is so complex. I cannot seem to find that much information.