WHOIS and RDAP both answer the question "who registered this domain and when?" - but RDAP is the modern replacement for the decades-old WHOIS protocol. WHOIS returns free-form text that varies by registrar and is hard to parse. RDAP (Registration Data Access Protocol) returns structured JSON over HTTPS with standardized fields.
The industry is transitioning from WHOIS to RDAP for good reasons: consistent formatting, secure transport, standardized responses, and support for access controls and internationalization. For lookups today, RDAP is increasingly the authoritative source.