Right, which means if someone in some other site linked to one of those videos and I clicked on it out of curiosity (or - more recently in my case - to pick it apart in yet another online argument), guess what kind of videos are going to spam my recommendations for months on end?

No, that's not how their algorithm works. I can't find the link now, but a former employee said that their algorithm is designed to keep you engaged with the site, and that typically involves directing you to more and more extreme versions of the previous videos. For example, if you were to watch a video about mainstream rock and continually follow YouTube's recommendations at the end of the videos, you'd end up at videos about death metal, or other niche genres. It does the same thing with politics; you can start out watching a Bernie Sanders video and end up at videos promoting full on socialism, or watch a video about increasing border security and end up at videos about immigrants ruining the country.

It's a correlative algorithm.

If a lot of people into video games and history watch X, it will recommend X to you. And it turns out that the community of active, engaged video games folk have a lot of chuds who are all over alt-right-to-fascist video content.