That dude is a machine. He wrote 4 secret novels during the pandemic on top of several regular ones.
Sounds almost like getting a rough requirements or ideas, then writing a more detailed specification and finally fully developed software that goes through rigorous testing and refinement.
It's quite different to how I thought most writers write. My assumption it's more straightforward with little preparation.
But many major authors produce a book every 1 to 5 to 10 years. Game of Thrones being the big example.