YouTube has a new video playback UI in testing in the Android app and it looks like they may be changing things up a bit. Since this is just a test not all users are going to be seeing it but there are a fair number of users who are likely already noticing the new look. The new UI change is relative to the way currently playing videos display within the application, as they now collapse to a smaller thumbnail view as a card style bar that sits across the width of the app UI.
The reasons for the new visual style likely factor a couple of things into the decision to implement the changes. While the video itself is smaller it's no longer just a thumbnail of the video like in the currently available picture-in-picture view that most users will still be seeing. Instead, the video is to the left-most part of the new card while the rest of the card features a white background with the name of the video, the video's channel, and a couple of playback controls which includes the play and pause button as well as the "x" button to close out the playing video when and if needed. Because the video is smaller users will see more room in the rest of the app UI and because the bar travels along the bottom edge of the app there is enough room for controls, this way users can pause the video if they want without having to expand the video back to its full page.
Google has also changed up the animation for the video playback when collapsing it. Currently the video collapses to the right bottom corner of the app, but with the new bar UI the video collapses to the left while the bar expands out to the right to make room for the video name and playback controls. Since this is just in testing there is no guarantee that Google will be making these changes available to everyone let along anytime in the near future, but users who are seeing are running with the 12.13.53 version of the YouTube so any users before that won't be seeing the new UI.