I am the primary editor. He writes a book, and sends it to me. After I get done with it, the book goes out to our beta readers, and their job is to find as many mistakes as they can. Those get fixed and DH goes through the book one more time then it gets published. Inevitably, readers will find at least half a dozen MORE mistakes within the first few days of the book going live.
He writes in a genre called Lit-RPG. It's relatively new (sort of, but not really). The genre generally revolves (sometimes loosely) around characters in a video game type of setting. MMORPGs are hugely popular model styles. Sometimes the character is trapped there, sometimes not. Fully immersive virtual reality is also a fairly common element. In a lot of ways, it's frequently a fusion of science fiction and fantasy. The sci-fi elements drive the mechanics, while the fantasy elements define the landscape.