Watching YouTube videos in picture-in-picture mode
YouTube has a picture-in-picture mode. With this enabled, the video displays in a small window in the corner of the screen. If you click and hold on the window, you can move it to anywhere on the screen. You can resize it as you can any other window. The advantage of being able to do this, is that you can perform any other task while it is playing. For example, you can open another tab, minimise the browser you are using, or open another browser/email program etc., and it will continue playing. This feature can be activated by clicking the icon shown below. This is shown on the right-hand side of a video when you play it or place the cursor on the window.
For devices other than a PC, google 'Android YouTube pip', for example.