Registrars provide the nameservers you give them to the TLD itself. So, resolution doesn't depend on the registrar's infrastructure unless you're using the registrar's nameservers directly.
It works like this: Root -> TLD -> NS -> NS -> ETC.
Root Servers are fixed and everything drills down from there: https://www.iana.org/domains/root/servers
A visual example of how the nameserver hops work starting from the root using the nameserver delegation view feature of our dns lookup tool: https://www.misk.com/tools/#dns/news.ycombinator.com@i.root-...
(Disclosure: I work @ Misk.com, an ICANN accredited registrar, our link above)