How to get iPhones to output 1080p Videos
iPhones do not support h264 at resolutions higher than 720p30.
If you use VP8 though, you will be using the software-based encoder, which will make the iPhone pretty warm/hot. It also only works only on newer iOS versions (iOS 14, for example).
In the future, iOS may support 1080p with the h264 hardware encoder.