YouTube has added HDR support for Apple's latest iPhones, the XS and XS Max.
The update to its iOS app allows users to to manually select HDR quality, however the Auto setting will automatically enable the option if it is supported by both the video and device.
Apple’s previous-generation iPhone X also includes HDR support, as do a number of Android smartphones. The highest resolution available on either of the new iPhone XS models is 1080p.
There is no option to watch videos in 4K.