Recommended resolution & aspect ratios
YouTube uses different video aspect ratios based on the platform and video format.
The YouTube app for Android mobile automatically adapts the player to the size of the video. Whether it’s vertical, square, or horizontal, the video will fill the screen. The vertical (portrait) video player matches the aspect ratio of the content - getting taller for square and vertical videos, and shorter for widescreen videos.
How vertical videos are displayed
Vertical videos will now display in the video player without black bars on the sides. See example:
A viewer can tap the square button on the bottom right of the video to enter into vertical full screen to view the entire video.