@stux You can most clearly see the effect by using anon ('incognito') browser mode over a VPN. This gives YT nothing to use as background for making suggestions that are unrelated to the current video.
Then access some video you're interested in, say music or renewable energy, and lo and behold... Youtube insists on suggesting the most awful non-related crap. They are probably a worse source of brainwashing than even Xitter.