It's impossible to prove a negative, like "Apple doesn't have a backdoor". One can prove the existence of a backdoor by reverse-engineering suspicious code or network traffic, but not the nonexistence without poring over every byte of machine code, and quite a lot of the hardware too.

This is not unique to Apple, it's impossible to prove any system is free of a backdoor, including Linux distributions (see: the xz backdoor, or "Reflections on trusting trust"), unless you hand-crafted your whole smartphone from raw silicon.