As another person who works at Google, I'm pretty sure we don't do this.
There's obviously no way I could know for certain. It's a big company. There could be some secret part of the organization that does shady stuff in the shadows.
As far as your concerns go, YouTube is still somewhat separated. I can't really speak for Google. I have almost no clue what YouTube is doing. But, this sounds more like a coincidence or actually how recommendation systems are designed to work.
Perhaps the content is popular in general, popular in the area, or what you think is unrelated -- statistically -- actually isn't. If you're watching videos with not many views... you are one of the only people to watch those videos. The recommendation system is going to recommend videos of people that watched that video before -- which you'll have a strong weight to. After just a few videos, your previous views will start to get significant weight. It's just how stats works.
Maybe Google is different (and this is not about ads), but I don't believe for one second that I'm being tracked on Youtube by anything other than fingerprinting (or some other method unrelated to IP and cookies).
I don't even particularly care that much, but it's still creepy, and I'm genuinely surprised to see denials because I thought fingerprinting was pretty standard these days.
There's obviously no way I could know for certain. It's a big company. There could be some secret part of the organization that does shady stuff in the shadows.
As far as your concerns go, YouTube is still somewhat separated. I can't really speak for Google. I have almost no clue what YouTube is doing. But, this sounds more like a coincidence or actually how recommendation systems are designed to work.
Perhaps the content is popular in general, popular in the area, or what you think is unrelated -- statistically -- actually isn't. If you're watching videos with not many views... you are one of the only people to watch those videos. The recommendation system is going to recommend videos of people that watched that video before -- which you'll have a strong weight to. After just a few videos, your previous views will start to get significant weight. It's just how stats works.