The registrar is the party you actually deal with: you pay it, manage nameservers and contact details in its control panel, and ask it for the auth code when you want to leave. Behind the scenes it talks to the registry of each TLD through a protocol called EPP. For gTLDs registrars are accredited by ICANN; ccTLD registries accredit registrars under their own rules.
Many domains are bought from resellers, such as hosting companies, that use a registrar's systems. In that case WHOIS and RDAP show the underlying registrar of record, which can be a name the customer has never heard of.
The registrar field in a lookup tells you where a domain is managed, and it is where abuse reports for a domain should usually be sent. It does not tell you who owns the domain or where the website is hosted.