I thought the way this works is that NSO Group determines there's a vulnerability in ios or android that lets them insert Pegasus, but soon after it gets deployed to spy on 1 person by a NSO Group client, Apple and Google eventually detects it and patches the vulnerability.
I don't think it scales to 'all iphones'.
There is a recent Frontline episode where NSO Group denies any knowledge of how Pegasus gets deployed once they sell the exploit system to a client, but that seems false to me since it is my understanding NSO Group runs a large amount of infrastructure to collect data from infected phones and to report data to the client, a multi tenant system.
I can't find a source for Snowden saying this either.