Programmatically read root CA certificates in iOS
The function SecTrustCopyAnchorCertificates
from Security.framework
that lets you retrieve root certificates stored in the system is only available on macOS. Curiously, it is one of the few functions (from set of related functions) that is not available on iOS. Deliberate, who knows?
I'm afraid it won't be possible to do an equivalent in iOS given the app ecosystem is sandboxed.
Without knowing your purposes, the usual approach for tackling this is downloading the apple root certificate from apple.com/certificateauthority and then storing it in your app for reading it.
Take a look this article for inspiring you as well.
PS: It might be possible to do this in an iOS device if it's jailbroken.