Things don't work like that.

First of all iPhones have more like 6-8GB of RAM, 1-2 of which are already taken up by the system and system apps. Add some resident apps and maybe 1-2GB are already taken. Then of course during peak times, which are predictable but not guaranteed, 5-10% is maybe available. So out of your 10GB estimated per device, you actually average maybe 3GB.

Similar story for the CPU and GPU.

Then, availability: dead battery, no cell reception, airplane mode, etc, etc.

And on top of that, in the context of battery charge and long term wear and tear, you're assuming people will just let you run Bitcoin mining nodes on them.

You need a really solid incentive for people to loan you end user computing power for legitimate reasons.