How did the Hashirama and Madara statues at the Valley of the End come to be? Were they built by traditional means (as they would've been in our world)? Were they created via some sort of ninjutsu that shaped the rocks that were there? Or was it erected by some sort of Doton jutsu (like some Doryuu Heki)?

I am not sure if there is some canonical answer to this, but I'd like to hear your thoughts.
From what I can tell, these statues are not protected or enchanted. After the Naruto vs. Sasuke battle at the Valley of the End, the status were damaged, and later repaired. I believe it was a reconstruction job, much like Leaf's faces on the mountain, or the Kage statues at the Sand.

Words of Madara
I honestly don't think Hashirama would give a damn enough to make statues, let alone self-repairing statues. He wasn't the sentimental type.