You can use picture-in-picture (PiP) to watch content across YouTube, with a YouTube Premium membership. If you don’t have YouTube Premium and are in the United States, you can still use PiP, but you can’t watch certain content, like music videos.
Note: If you’re outside of the US, you’ll need a YouTube Premium membership to use PiP with any content on YouTube.
Note: Make sure you have YouTube Premium and you're on iOS 15.0 or later to use picture-in-picture on your device.