The iPhone is a series of overpriced under-powered smartphones made by some fruit company. These devices are unique in their lack of support for modern web standards. They are also problematic to deal with when it comes to really basic tasks like copying files to and from the devices; you can't copy files to them.
The screen resolutions you need to consider when making web pages for mobile devices are:
- iPhone 5: 640x1136
- iPhone 6: 750x1334
- iPhone 6 plus: 1242x2208
- later models have higher resolutions
Thus; the standard resolution to target is either 640x1136 or 750x1334. Later models will look fine if those resolutions look fine.
The iPhone can not play VP9 video and it can't play Opus audio. This limitation is intentional. Thus; you will need to provide a MP4 copy of videos served by websites targeting iPhone users. And that would be all of them except this one given those devices market-share. Most people using this site have Android phones but this site is rather specialized and not an example of the web in general.
The iPhone 4 can be used to make phone calls if you hold it the "right way"