A lot of authors are commissioned though, basically given a brief of what to write. More often than not in more commercial franchises though, e.g. Marvel, Warha

A lot of authors are commissioned though, basically given a brief of what to write. More often than not in more commercial franchises though, e.g. Marvel, Warhammer, video game adaptations / tie-ins, Minecraft, Manning books, etc. But that goes into the realm of commercial media, vs "artistic" media where authors can just do whatever they want, and maybe they'll find a publisher and get successful.