The videos I'm thinking of that someone sent to me are way outside of what I would normally watch, so seeing that exact video in recommendations and other videos from the same channel immediately afterwards on the home page can't be a coincidence. But most of the time what I watch doesn't affect recommendations, only the odd time.

In addition to curating what I want to see with upvotes/subscriptions, I also use the "Not interested" and "Don't recommend channel" buttons heavily. For some reason YouTube always recommends those stupid "meditation music" videos, even though I have never watched one and I always block those channels. Honestly, YouTube's recommendation engine is shite; even when I tell them exactly what I like, it still recommends absolute tripe, and most of what I discover comes from external (personal) recommendations.