"relay.dnsp.co" is the one option here that performs a real lookup. Append it to any name — "google.com.relay.dnsp.co" — and we strip the suffix, ask an ordinary recursive resolver, and hand you back what it said.
Every other test on this site answers with something deliberately synthetic. This one exists so you have an honest answer to compare against, obtained through exactly the same path. If a name resolves correctly through "relay" but not directly from where you are sitting, the difference is somewhere between you and us rather than in the name itself.
It relays whatever record type you ask for, not just addresses — A, AAAA, TXT, MX and NS all pass through. Bear in mind that a name using geographic routing will answer according to our location rather than yours, so large sites like google.com will often return different addresses here than they do for you. That is the name behaving normally, not interference.
Asking for "relay.dnsp.co" on its own, or for a name that would refer back to itself, returns SERVFAIL rather than looping.
prompt> dig +short example.com.relay.dnsp.co 172.66.147.243 104.20.23.154
prompt> dig +short example.com.relay.dnsp.co NS hera.ns.cloudflare.com. gordon.ns.cloudflare.com.
prompt> dig nosuchname.relay.dnsp.co ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 39937
prompt> dig relay.dnsp.co.relay.dnsp.co ;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 27939