Windows phone. Originally Microsoft put out a number of apps as web wrappers, but the mobile YouTube site kind of awful. So Microsoft wrote a YouTube app of their own that was actually kind of great and allowed you to download videos and play audio in the background and basically actually work right. Google threw a fit and basically made Microsoft delete the app.

Windows central still has a bunch of articles from the time up.

https://www.windowscentral.com/search?searchTerm=Phone+YouTube