Unlike Picture-in-Picture in Safari (below) it seems that users will not be able to move videos on the screen or to show up on other software / windows.
There is currently no official statement from YouTube about testing new features, so we still have to wait until it is released widely. Because it is already on mobile and also increases the user experience very well, helping to watch videos while searching, the Picture-in-Picture appearance on the desktop is not surprising.
If you are using macOS from Sierra onwards, you can enable this feature on Safari browser by right-clicking on the watching YouTube video. Ignore when a menu appears and right-click again. The second menu will have the option Enter Picture in Picture. Click on it and the video will turn to the corner of the screen, and continue to run when you switch to open another browser or software.
To stop the video, click the multiplication mark on the left, and click the box with the arrow will switch to normal video view.
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