If iPhones had flaws in the encryption or security, they WOULD be exploited and monetised.
A zero day remote attack on an iOS device is worth so much money that you have to be _really_ ethical as a hacker not to sell it and report it to Apple for a small reward instead.
The last time one was deployed "publicly" was against Jeff Bezos (or his wife) - one of the top10 richest people in the world anyway. And then it was patched for everyone.
Apple owns the code and all it's visibility - you're not allowed to see it.
They don't need a 0 day to compromise your device, they can just choose to do it at any point. You're simply trusting they don't.