The wall wasn't built all at once, it was built by various Chinese dynasties over a period of centuries. The first sections of the wall were built over 2700 years ago, with the most recent ones being 400 years old. The older sections are quite a bit more primitive (and smaller) than the newer sections (in some cases just stones packed on each other), but even the wall sections built 400 years ago are still a stone wall that requires no advanced tooling. It's a very long wall, but given the fact it was built over thousands of years by multiple different Empires doesn't really make it miraculous or anything.