If you don’t have Chrome, download it now from the App Store. You must be using iOS 14 or later to use Picture in Picture.
To close the window, tap it once to bring up the controls, and then tap the X at the bottom-left corner.
If you don’t subscribe to the Premium version of YouTube, you can still play YouTube audio in the background while you do other things if you use YouTube in the Safari web browser.
The audio will stop playing, but you’ll be able to start it again in just a moment.
The next video in the playlist will not play automatically. You’ll need to return to the YouTube page to play another video, re-open the Control Center, and then press Play.