Unbound will be used.

Install the recursive DNS resolver

sudo apt update
sudo apt install unbound

/etc/unbound/unbound.conf.d/pi-hole.conf

server:
    # If no logfile is specified, syslog is used
    # logfile: "/var/log/unbound/unbound.log"
    verbosity: 0
 
    interface: 127.0.0.1
    port: 5335
    do-ip4: yes
    do-udp: yes
    do-tcp: yes
 
    # May be set to yes if you have IPv6 connectivity
    do-ip6: no
 
    # You want to leave this to no unless you have *native* IPv6. With 6to4 and
    # Terredo tunnels your web browser should favor IPv4 for the same reasons
    prefer-ip6: no
 
    # Use this only when you downloaded the list of primary root servers!
    # If you use the default dns-root-data package, unbound will find it automatically
    #root-hints: "/var/lib/unbound/root.hints"
 
    # Trust glue only if it is within the server's authority
    harden-glue: yes
 
    # Require DNSSEC data for trust-anchored zones, if such data is absent, the zone becomes BOGUS
    harden-dnssec-stripped: yes
 
    # Don't use Capitalization randomization as it known to cause DNSSEC issues sometimes
    # see https://discourse.pi-hole.net/t/unbound-stubby-or-dnscrypt-proxy/9378 for further details
    use-caps-for-id: no
 
    # Reduce EDNS reassembly buffer size.
    # Suggested by the unbound man page to reduce fragmentation reassembly problems
    edns-buffer-size: 1472
 
    # Perform prefetching of close to expired message cache entries
    # This only applies to domains that have been frequently queried
    prefetch: yes
 
    # One thread should be sufficient, can be increased on beefy machines. In reality for most users running on small networks or on a single machine, it should be unnecessary to seek performance enhancement by increasing num-threads above 1.
    num-threads: 1
 
    # Ensure kernel buffer is large enough to not lose messages in traffic spikes
    so-rcvbuf: 1m
 
    # Ensure privacy of local IP ranges
    private-address: 192.168.0.0/16
    private-address: 169.254.0.0/16
    private-address: 172.16.0.0/12
    private-address: 10.0.0.0/8
    private-address: fd00::/8
    private-address: fe80::/10

sudo service unbound restart

dig pi-hole.net @127.0.0.1 -p 5335

dig sigfail.verteiltesysteme.net @127.0.0.1 -p 5335
dig sigok.verteiltesysteme.net @127.0.0.1 -p 5335

Navigate to Tools → Settings → DNS.

In Upstream DNS Servers:

• Untick any pre-defined servers.
• Change the Custom 1 (IPv4) to 127.0.0.1#5335.
• Click Save.

https://docs.pi-hole.net/guides/dns/unbound/