Using the Firefox browser?

This function works out of the box in the Google Chrome and Microsoft Edge browsers. But in Firefox you need to use an add-on called enable pip add-on. It's easy. Here's a very short video showing how to do that.

Safari's PiP button is hidden in the address bar
If a video is playing in Safari, go to the address bar at the top of the window, right-click the blue audio button to the right of the website URL in the address bar and click Enter Picture in Picture. The video will then automatically move to a floating window.

Apple iPad

Enable picture-in-Picture (PiP) on an iPad by following these steps:

• Go to Settings
• Select General
• Select Picture-in-Picture
• Toggle Start PiP Automatically to turn it on
• Go to the app settings
• Select General
• Toggle Picture-in-Picture to turn it on 

PiP allows you to use other apps while watching a video or using FaceTime. You can use PiP on the following iPad models: iPad Pro, iPad (5th generation and later), iPad Air 2 and later, and iPad mini 4 and later.