These buildings were massive and they're often made of conrete that actually self heals, similar to roman concrete. (Which we only recently "rediscovered" to make as well, iirc!)

I don't know who built them. Some people suspect it was "Tartarians" (because of a supposedly wide spread empire of that name on ancient maps), some suspect we're in the little season and that these buildings are remnants of the millennial reign.

I'm not entirely sold on any of these hypotheses but I'm highly sceptical of the common historic narratives were fed everywhere and I'm sure there was a lot more peace and abundance for a few hundred years at least than we're led to believe. It all points to some reset around the 1700-1800s or so.

Check out Mind Unveiled, Lucius Aurelius, My Lunch Break or Robert Sepher's video about that on YT for a high level overview to get deeper into it.