See https://uk.pcmag.com/news-analysis/120580/how-a-vpn-review-s... for discussion of major trickage.
I'm no SEO guru, but I suspect that some of those VPN services created numerous clones, which all linked to TheBestVPN.com, and so improved its ranking. For example, ExpressVPN had at least 128 clones. Such as expressvpn..., buy-express-vpn-..., get-xpress-vpn...., and xpress-vpn.... I used myip.ms to get hosting information, and they were linked. Also, I bought subscriptions from a few of them, and they all provided working ExpressVPN apps, with the right certificates. And I found no evidence of affiliate codes in the traffic.