My lunch break yt channel has TONS of videos on this stuff, if you're curious I'd browse through his top videos.

The gist is that logistically, building these gigantic, perfect, marble buildings doesn't make any sense with 1800s hammer chisel and donkey carts, and population at the time (everyone was just master builders I guess).

Coupled with suspiciously repetitive fire narratives (you'd be shocked how often stone buildings "burn to the ground") and insane <1 year build times, the general idea is that anything pre-1800s is fake history and "something happened" to the old world people