So it feels like the way YouTube's algorithms work re suggested videos is heavily related to "what other videos did the people who watched this video watch?". Now imagine you have a video that expresses some conspiracy theory. That video is watched by a bunch of people who are into conspiracy theories. Then you come along, and YouTube now recommends you videos that the people who watched the same video as you just watched. And bam you get sucked toward those others who were interested in that video.
Now this does work in the other direction, where someone with more normal viewing habits watching a video will steer the conspiracy theorists toward the types of videos they watch, but the size of the pools means it's the polarizing direction that would have a greater effect.
It's worse than that. YouTube tries to maximize engagement, so it will preferentially recommend videos watched by highly engaged viewers. Guess what category of viewers are highly engaged? Indeed, conspiracists, polemecists, etc. Imagine walking into a college campus and asking the registrar for some recommendations on classes to take and them telling you to go listen to the crazy religious nut shouting from atop a milk crate, because they were so much more engaged with their material than anyone else on campus.
Yes, recommenders take content you've interacted with and map it to other content that they see links to (people who hit Like on video X also hit Like on these other videos).
But what I'd point out is that I see YouTube respecting _all_ the content I see, rather than just the most recent one or just right-wing content, as some commenters here are alleging. For example my feed usually includes videos from Vox as well as Fox on politics, alongside video game reviews, alongside a NASA stream, alongside woodworking, and so on.
Now this does work in the other direction, where someone with more normal viewing habits watching a video will steer the conspiracy theorists toward the types of videos they watch, but the size of the pools means it's the polarizing direction that would have a greater effect.