Every page view starts with a lookup. The browser asks a recursive resolver; that resolver asks (or remembers) your authority. The TTL on the last answer decides how long the world will keep using it.
Four actors, one question
The stub resolver lives on the device. The recursive resolver caches and walks the tree. The parent (TLD or registrar) holds NS delegation. The authority — Cloud DNS if you host the zone there — returns the record.
GeoDNS and health checks run at the authority. They cannot rewrite an answer already sitting in a recursive cache.
TTL is a contract with strangers
You publish a TTL. Many resolvers honour it. Some clamp it. Designing as if every resolver will forget in one second is how people get surprised.
Still use a low TTL on records you might move. Health-aware answers help even when a cache is sticky: you are less likely to keep publishing a dead target.
Where CDN fits on this path
DNS returns an address or hostname. The CDN is what that name delivers. Flattening the apex to Cloud CDN is a DNS record strategy sitting in front of a delivery strategy.
If lookups fail, the CDN never gets the request. Protect both.