youtube-dl can fetch a video via its ID, the part after the "/watch?v=<ID>" in the URL. Combined with a video player, such as vlc, mpv, mpc-hc or similar, you can stream youtube videos without viewing from the youtube website. There are scripts available that make this less DIY, such as ones that launch your video player with youtube-dl when you load a page on youtube, though I do not have any of those on hand to show you.
The question was more about discovery than how the tool worked... I'd imagine he doesn't watch much YouTube but just uses youtube-dl when the videos are found on other sites.
I watch a bit of YouTube, but I only use YouTube-dl to doownload videos I really like - like abridgements or YTPs - stuff that can easily be copyright notice'd down
