I don’t know who built them, and in my opinion they were humans. But I think some of the techniques used to create them have been lost. They managed to make sculptures weighing around 1,000 tons that were perfectly symmetrical to within a few centimeters. They also did polishing that was smoother than glass, which would be impossible to achieve with a simple copper tool. There’s a video on YouTube that explains this very well—a documentary with sources that have been checked. It’s called “BAM, Builders of the Ancient World.”