YouTube has added HDR support for iPhone XS and XS Max. This new change was first noticed by users after one of the most recent YouTube app update. HDR was earlier said to be available only on the iPhone X on iOS.
HDR video includes dynamic range, higher peak brightness and also wider color gamut as compared to standard video. Also support is required on both hardware and software level. There are some devices which also fake it using the software.
Resolutions above 1080p are still not supported on iOS. Currently, higher resolution options are only available in its own VP9 video codec by Google. However, the same is not supported on Apple devices.