The rise of the 18:9 display has been incredible this year. The trend can be seen on high-end handsets like the Galaxy S8, Galaxy S8+, and LG's V30, along with some of the more affordable models available like the LG's Q6, Honor's 7X, and others.
This extra screen real estate is great on paper, but if the app you're using doesn't support it, it just doesn't make sense. This was the case for YouTube, where a majority of content displayed black bars on the right and left-hand sides of the display when watching content on a majority of phones with an 18:9 display. This isn't the fault of content creators mind you, where 16:9 is considered more traditional, but a simple pinch-to-zoom option would have been nice.
Luckily, those that own an LG V30, Samsung Galaxy S8, and possibly other handsets, will be happy know that the zoom feature is now available in the latest update to the YouTube app. With version 12.44, owners of the previously mentioned handsets will be able to zoom into their favorite YouTube videos to fill the whole screen of their phones. This update could enable the feature on other 18:9 devices, so if you want to check to see if it applies, be sure to download the latest update.
Source: Android Police