How could one verify that? like for compliance audit?
Introduction to Apple security assurance
As part of our commitment to security, Apple regularly engages with third-party organizations to certify and attest to the security of Apple’s hardware, software, and services. These internationally recognized organizations provide Apple with certifications that align with each major operating system release. …
I can't think of any entity I would trust with securing truly sensitive information. For important stuff, do it yourself. For simple things, including bank accounts and such, I see no issue with trusting Apple.
I don’t know I buy the “for truly sensitive stuff do it yourself” line. That’s like saying for the truly lethal substances handle them yourself. Most people aren’t more skilled than the apple security folks. You’re almost certainly going to screw up your encryption or leave some vulnerability unpatched or unknown. Frankly I consider my iOS devices to be some of the most secure systems I have access to, and reading through their security documentation has informed that opinion.
The cynical view, of course, is that Apple's incentive and the Third Party's incentive can become very much aligned for the right amount of money.