If systemd-resolved is actually being used, /etc/resolv.conf will list only 127.0.0.53, and to find out what systemd-resolved itself is using one has to visit t

If systemd-resolved is actually being used, /etc/resolv.conf will list only 127.0.0.53, and to find out what systemd-resolved itself is using one has to visit the resolved.conf configuration files, which is what falcolas is apparently talking about. But then if one is using, say, dnscache or dnsmasq or MaraDNS or BIND or PowerDNS or any other such software, one has to do the same thing. /etc/resolv.conf will list simply 127.0.0.1 or some such, and one has to go further to the configuration files of those softwares to find out what they are using for resolution/forwarding. DNS softwares having their own configuration files is normal. falcolas is apparently getting at the fact that systemd-resolved uses the same style of configuration as the other systemd programs do: files and drop-in directories in /etc/systemd, /run/systemd, /usr/local/lib/systemd, /usr/lib/systemd, and (if configured) /lib/systemd, meaning that one has to read all of this together.