Are you asking outside the context of the home surveillance cams? just whether it's possible to prevent backdoors to your iphone camera?

IME you have no control over the baseband chip of a cell phone, no reason to trust it's not enabling its camera or microphone at any time. I have a flip phone which comes in a non-camera version. I have an iphone without a SIM I can connect to its hotspot if I need to do something smartphoney.

Yes, I'm asking outside the context of home surveillance. I'm just curious how "paranoid" people deal with smartphones. I'm paranoid myself, but I'm also lazy ...

As a paranoid and lazy person, I just pick my battles. No amount of fiddling with hardware security keys and secure computing is a reasonable protection against rubber hose cryptography.