Ask HN: How YouTube made their comments good?

A few years ago, I remember that YouTube comments were the epitome of shithole, they were full of hatred.

And now they're usually pretty good, under most of the popular videos it's just good-hearted jokes. There can be a factual correction, but it's rarely a shithole that you can see on other social networks (e.g. Twitter).

There certainly weren't any active moderation on scale, so it must be their design, incentives and ranking algorithms. So, the question is, how they did it?