The new button is currently in version 19.5 but seemingly not widely available yet.

YouTube is testing a pop-up “Play Something” button that will play a random video for you. As spotted by 9to5Google, the new ...

When tapped, it uses the YouTube Shorts player to start a random video, whether it's a Short or a standard YouTube video, ...

YouTube is testing a floating "Play something" button on Android, randomly picking videos to play. It uses the Shorts player ...