Hi Neil,
If you are recording on a newer iPhone, it should have 4K as an option, if you are in the camera mode you can adjust that setting in the top right hand corner of the camera app. If its not there, it is in the settings. From there you can select your desired resolution, whether its 720p, 1080p, or 4k. I believe they have varying frame rates as well. All of which should be great for web use content!
I hope that helps
Best,
Thomas
Cinematrocity Video Production
https://cinematrocity.com