Two public registers describe a domain without touching its servers. RDAP, the successor of WHOIS, is what the registry knows: registrar, registration and expiry dates, status codes such as a transfer lock, nameservers, DNSSEC delegation. Certificate Transparency is what every certificate authority had to log since 2018: every host name that ever received a certificate. Read together they answer questions a site owner rarely asks until it is late: when does the domain expire, is it locked against transfer, and which subdomains still exist somewhere because a certificate was once issued for them.
From this server: one RDAP query via rdap.org, which redirects to the responsible registry, for registries without RDAP a whois query, and one query to crt.sh for the certificate logs. Nothing on the domain itself is contacted. The query is protected by reCAPTCHA v3; the calling IP address and the domain are stored for one hour to limit the rate.
Limits worth knowing: RDAP data depends on the registry, and some (such as .de) publish no RDAP, so whois text is parsed instead, with fewer fields; crt.sh answers are cut at 3 MB, which for very large domains leaves out old certificates; a name in the logs proves a certificate was issued, not that the host still answers; and registrant details are not shown, because registries no longer publish them.