When playing a video in Safari on your Mac, you can Control+click on it and select Enter Picture In Picture. But this doesn’t seem to work on YouTube videos. When you click on a YouTube video, you get a special context menu from YouTube instead of the default macOS one. The trick is to leave this context menu there without selecting anything, and then Control+click the video a second time to get the macOS context menu with the Enter Picture In Picture option.
Mac Video Tutorials and Tips for Apple Users
Nice tip! Works on other browsers as well (well, it did on Brave anyway).