DNS and domains
RDAP and whois
The two ways to ask a registry who holds a domain: whois, the old text protocol on port 43, and RDAP, its JSON successor over HTTPS.
Whois is the original lookup: connect to the registry's server on port 43, send a domain name, get back free text in whatever format that registry uses. RDAP replaced it for most registries with a JSON API over HTTPS, a standard format, and a bootstrap file at IANA that says which server to ask for which extension.
Both return roughly the same facts: registrar, dates, status codes, nameservers, whether the domain is signed. Since the GDPR, the holder's name and contact details are hidden by almost every European registry and shown only on their own website, often after a captcha.
Not every registry offers RDAP. For .lu, .be and .eu you still need whois on port 43, and the three registries rate-limit it per address. For .com, .net, .org, .fr, .nl and most others, RDAP is the way.