iPhone X, but meanwhile, developers are working hard on updating their apps to support the mold-breaking 2:1 display of the device, which is a first for Apple.
One of the latest top-tier apps to score support for the iPhone X's intricacies is none other than YouTube, which now support pinch to zoom for a fullscreen experience alongside a regular letterbox mode that doesn't make full use of the display real estate.
Once you update the YouTube app on your iPhone X to v12.43, you will be able to simply pinch in on any video and it will fill in the whole screen. This way, however, some content will be cropped from the bottom, top, and one of the sides of the phone (thanks, notch). That's why even after you update the app, YouTube will default to the more traditional letterbox mode which doesn't hide away content.
The update should be rolling out right now.