Earlier quoted context omitted.
I clear my YouTube search and watch history about once a week. Partly because of privacy, but also because a single binge of, say, metal casting videos does not mean I want them recommend ever again in the future.
They use browser fingerprinting and/or IP addresses as well. Even on a brand new browser session, doing something even slightly related to the previous session brings back its entire recommendation history.
Are you sure that it does, and that it's not just a case of "hey we've never seen this person before, but they watched X, let's immediately start with recommendations Y and Z because other people who watched X were engaged with it?"