Authoritative DNS is the service that owns your zone. When a recursive resolver asks what www.example.com is, the nameservers you delegated are supposed to know. If they are slow, attacked or stuck on a high TTL, nothing else on the site matters.
Why unicast nameservers are a single path
Two nameserver hostnames that share one network path still fail together. Anycast advertises the same authoritative service from 210+ servers so resolvers use a nearby available path, including during DDoS.
That is the DNS equivalent of a CDN: the record is the same, the place it is answered from is not.
Records are policy, not just addresses
A and AAAA are the start. GeoDNS, weights, subnet policy, health checks, CNAME flattening, DNSSEC and a one-second minimum TTL are how DNS becomes part of application architecture.
Registrar DNS is fine for a parked domain. It is a poor control plane for a shop or an API that must move.
Delegation is the cutover
You import the zone, prove records, then change NS at the parent. Mail and verification TXT should be in the imported zone so they do not vanish when NS moves.
Do not mix this up with pointing a single A record at Cloud CDN. That is a record change. Delegation is who is allowed to answer the zone.