For those unfamiliar with the feature, it debuted on iOS devices first where the player could automatically adjust itself to content in different aspect ratios. This means that regardless of how the video was captured or your device’s display size, the dynamic video player means that YouTube will try and cut out as much of the black bars as possible to give users a full screen experience.
https://twitter.com/YouTube/status/978315887957696512
As you can see in the tweet below by YouTube, this means that even if you were to scroll the video will expand and change its shape to fit your display accordingly. The feature rolled out for iOS devices last December and it looks like it has finally made its way onto Android. The changes should already be live so if you have YouTube on your Android phone, you can check it out for yourself. If you don’t see the changes already, not to worry as we expect it could possibly be rolled out in phases and it should get to you eventually.