It's not really fair to call them boulders, they're custom statues with rounded bottoms that are carefully balanced, but they can be moved. The purpose of the project is to show that it's possible to build monolithic stones like those on Easter Island and at Stonehenge and move them by hand.

The embedded videos are quite amazing!

https://gizmodo.com/researchers-made-25-ton-boulders-they-can-move-by-hand-1834106230