YouTube improves the video experience on the iPhone X and the iPhone 8 (Plus): As reported by The Verge, Google is currently ensuring that owners of these devices can now watch HDR videos on YouTube. This has already been possible on Android for several months .
Google seems to be releasing HDR support for iPhone X, iPhone 8 and iPhone 8 Plus on the server side. An update of the YouTube app is therefore probably not necessary. High Dynamic Range (HDR) affects videos on color, brightness, and contrast. The higher dynamic range makes the colors more saturated and the videos brighter. However, the YouTube videos themselves must also have been recorded accordingly. By far not all clips on Google's platform benefit from the HDR support for iPhone X and iPhone 8 (Plus).
HDR even at lower resolutions
If you want to get an idea of what HDR videos look like on the iPhone X or on the iPhone 8 (Plus), the quickest way to do so is via the search function of the YouTube app: Just enter "HDR" in the search bar and already you get a selection of HDR videos. If you play one of them, your iPhone should automatically switch to HDR mode.
You can also activate the HDR mode manually by tapping on the three dots in the top right corner and then on "Quality". Here you can also change the resolution - such as when the video should jerk at a high resolution. You do not necessarily have to set the maximum resolution, because the benefits of HDR anyway primarily affect colors, contrast and brightness.