I have no idea how you would begin to demonstrate that a body of source code as big (in both lines of code and length / size of the development project) as iOS, Android/Replicant/Lineage, or any other modern operating system contains zero backdoors, especially when your threat model involves the software authors helping you over a period of many years to weave those backdoors into the OS and obfuscate them as much as necessary.
I also have no idea how to audit a physical phone I have received from an Apple Store to make sure that the hardware and firmware faithfully implement what they are supposed to be implementing.
I have no idea how you would begin to demonstrate that a body of source code as big (in both lines of code and length / size of the development project) as iOS, Android/Replicant/Lineage, or any other modern operating system contains zero backdoors, especially when your threat model involves the software authors helping you over a period of many years to weave those backdoors into the OS and obfuscate them as much as necessary.
I also have no idea how to audit a physical phone I have received from an Apple Store to make sure that the hardware and firmware faithfully implement what they are supposed to be implementing.