Apple is one of the exclusive brands that have their own way of getting things ready. Each of their technologies has some unique features including the iPhone. Apple iPhone has its own codecs when it comes to applications and they didn't make an exception for YouTube. But, it doesn't mean you cannot have access to the application or have to compromise the picture quality.
YouTube apps usually adapt to the situation very quickly and even update themselves exclusively for iPhone users. Last year in September, YouTube updated its iOS app and added HDR support for the iPhone XS and XS Max. This year also the app added up HDR support for iPhone 11, iPhone 11 Pro and iPhone 11 Pro Max.
In order to avail the YouTube HDR support for the iPhone line up, YouTube has updated itself recently. You can download the YouTube app from the App Store and check out some videos. To check if you are using the HDR or not, simply tap the three dots in the top-right corner of a video. Then click on 'Quality' and check if HDR appears next to the selected output or not. If it doesn't show HDR then select any YouTube HDR quality option and enjoy your high YouTube definition video.
However, the 4K videos are still not made available on the iPhones in spite of huge user demand. We hope that YouTube will look into it and soon reach a decision to satisfy its users.