Hey! Thanks for the clarification (in Many queries to these domains). I came here via google, because I observe these quad9 requests, too.
They originate from my Apple devices (iPhones, Macs, TV).
Does this mean, they circumvent my Pi-Hole? Why would clients request the IP of dns.quad9.net?
Thanks. 
Maybe some apps on these clients have them hardcoded.
That means the clients use the Quad9 DNS for DoH queries instead of the local Pi Hole?
How unfriendly of them.
Thank you for the response.
I will try to block these requests and check if anything breaks. 
This thread might help with deciding on further actions: Blocking DNS-over-HTTPS (DoH)
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