WhatsApp is currently testing a feature that'll allow you to view YouTube videos directly within the chat window, eliminating the need for the app to launch the YouTube app for playback.
The feature enables playback to be launched in a picture-in-picture window, while also offering functionalities like pinch-to-zoom, full screen view mode, and hide ongoing playback to continue reading messages in chat.
However, the playback will stop if you switch to some other chat. Reportedly, the feature is currently only available on iOS, with supported devices including the iPhone 6 and above. No such tests have been noticed on the app's Android and Windows Phone clients.
If testing goes fine, iOS users will be able to enjoy WhatsApp's new in-app YouTube playback soon.