List of used DoH servers:

Server 1: https://dns1.us.newpangea.de/dns-query

Server 2: https://dns.kusoneko.moe:9443/dns-query

Server 3: https://dns.furrydns.de/dns-query

Server 4: https://doh.kekew.info/dns-query

Server 5: https://dns.sev.monster/dns-query

Server 6: https://cloudflare-dns.com/dns-query

Server 7: https://dns.quad9.net/dns-query

Server 8: https://dns.opendns.com/dns-query

Server 9: https://dns.google/dns-query

What are these?

Pi-hole is not a DNS server by itself. Pi-hole (the backend DNS software) is an intermediary. It stands between you and one of the above servers and filters out undesirable domains, such as ads.

What these domains have to do with it is explained better here.