I would guess most are written by humans. I know a lot of affiliates (mostly gambling but also finance and shopping) and most of their stuff is produced manually. There is some more automation these days but the industry is surprisingly manual.
I have the impression that they use ghostwriters that summarise a couple of Amazon reviews, gain some superficial familiarity with the subject (talking points), but have no real experience with it. I've seen many of such sites (mattresses, bike accessories, VPNs, certain apps, etc.) that seem superficially plausible, but here and there betray profound ignorance of the subject matter.