It's always a game of cat and mouse, but NSO had a quarter billion USD in annual revenue in 2020. They are clearly providing highly effective spyware to governments around the world. It wouldn't surprise me if they have that many zero day, zero click exploits that they can always get in to a phone. We're talking nation state espionage here... they probably have insiders at Apple and Google who introduce subtle unnoticeable bugs in core OS stacks.

How does Apple discover their exploits? I'm sure they keep some around for extremely high value targets.