Algorithm, Search & Distribution
Video category
Also called: category setting
It sits in the upload details under the expanded options, and it is one field with one choice from YouTube's own list. The list is short and broad, so most videos have an obvious home and a handful sit awkwardly between two.
Expect little drama from it. Creators periodically decide the field is a hidden growth lever and start switching categories on old videos hunting for a lift, and nothing published supports that. Pick the honest fit, set it in your upload defaults so it stops being a decision every time, and give the attention to the title and thumbnail.