The et's who have come here and lived among us at different times in history designed the monolithic buildings and pyramids. They trained people in construction trades, higher math, and some learned to pilot utility construction craft.
They et tech included diamond cutters, laser craft that were programmed to engrave stones, and finish the stones.
They used other craft similar to helicopters, but high tech, to fly stones from quarries. Anti grav trasnponders were placed on the stones, lifted, and flown to site by these cool choppers.
Trained construction people on the ground helped finesse the stones into place that were lifted by anti grav craft, after the stones around the placement were treated with a chemical that created a heat bond.

The construction of these monolithic sites was also aided by some present day time travelers as well, and surplus equipment. But, that doesn't apply to all sites.