YouTube is updating the new look and feel for the full-screen view on the mobile app. This interface allows users to click Like or Dislike a video more quickly, view comments and share videos, and has an additional button for you to browse more related videos. The old version has hidden most of those features, users have to swipe up to access.
These changes are only visible when you are watching in full screen mode, in portrait view, the YouTube interface is almost unchanged.
New interface
Old interface (after swiping up to access buttons)
Comments can now also be easily accessed while watching videos in landscape mode. Before the new interface, you had to tap the comment section while in portrait mode to open the comment, then switch to full-screen mode. You can now access them by clicking the comment button located next to the Dislike button.
The new interface will be available on both iOS and Android and will begin rolling out automatically to users on Monday, according to Allison Toh, a Google spokesperson. However, it seems that this interface will not be updated to all users at once, so if not, please wait patiently.
Reference: TheVerge