Aren't all these public DNS getting unencrypted requests, so I assume ISPs snoop the domain lookups already, regardless of Google/Cloudflare/OpenDNS/Yandex doing so.
That's the point of using DoH, to avoid sending unencrypted DNS requests so your ISP can't spy or intercept those requests. If you are using unencrypted DNS from Google/Cloudflare/etc you are just adding one more party that can see your requests. If you use DoH, in theory, you are replacing who can see your requests. In practice your ISP can still know what websites you visit thanks to unencrypted SNI or if the domain you are visiting is the only one on that IP (and probably other techniques I'm not aware of). There are many more variables than just DNS requests so if you really don't want your ISP knowing what websites you visit you have no choice but to use a VPN or Tor.
