At first, I assumed that this was a clever technique using something flexible like a net or tubing. After a bit of digging, I found a similar tower roof on a different build by the same person:
Someone had asked about the roof technique on that build, and Markus included a photo looking up the inside of the roof structure:
The build makes use of round elements of different sizes with elements of an appropriate length clipped around them like spokes. This is a great technique that delivers a nice result if you have loads of parts and patience available.