WHOIS is the long-standing protocol and service for looking up registration data about a domain or IP address — the registrar, registration and expiry dates, name servers, and historically the registrant's contact details.
Since GDPR and related privacy rules, registrant details in WHOIS are frequently redacted behind a privacy proxy, so the registrant is often hidden. The registrar and its abuse contact, however, remain available — which is what you actually need to file a takedown.
WHOIS is being superseded by RDAP, which returns the same data in structured JSON and is the preferred lookup for automation. See the guide on reporting a fake domain to a registrar.