ranking

Accuracy is meaningless for ranking, the whole point is the order. The real question behind the question is whether your relevance is graded or binary and whether one good result is enough, because that picks the metric. Then the trap: your offline NDCG can climb while online engagement falls.

New courses launch every week and most browsers are signed-out or brand new, so the interesting half of this problem is what you recommend when you have almost no behavioral signal. The other half is not optimizing engagement into a clickbait catalog.

The whole product lives or dies on a feedback loop measured in seconds: a swipe is a label, and the next video has to reflect it. The hard part is exploring enough to discover what someone likes without trapping them in a filter bubble or amplifying junk.