It simply uses the publicly available embed API and a wrapper of the website itself:
Introducing Juno for Apple Vision Pro
YouTube is probably one of the parts of the internet I consume the most, so I was more than a little sad when YouTube announced that they don’t have plans to build a visionOS app, and disabled the option to load the iPad app. This leaves you with Safari, and the website is okay, but definitely...christianselig.com
Which won't stop them from stopping him, if they end up wanting to