The number of people with access to the private keys that sign iOS updates must be very limited - I wouldn't be surprised if you needed at least two people actively involved in signing every update.

On top of that, Apple is heavily siloed, and somebody working on the Calendar app won't have information on the operation of Secure Enclave, the chip that deals with authenticating fingerprints, passcodes, Apple Pay etc.

So it would be more like "Hey tech dude, can you sneak a change to the compiler your company uses to build iOS, to have it compile a backdoor into iOS, then surreptitiously login to the relevant machines and place your new compiler?"