What RDAP is
RDAP is the modern replacement for WHOIS: domain registries and the regional internet registries (RIPE, ARIN, APNIC and others) publish data over it in one format. Personal owner contacts are usually hidden under privacy rules, but registration and expiry dates, the registrar and the name servers are always shown.
Some country zones do not support RDAP yet; for those you need the registry’s own WHOIS server.