You can go too deep on this one and find people who probably think it’s a conspiracy or something lol, but the gist is that YouTube used to recommend very very odd videos to kids. People picked up on this and made odd videos. It became like a feedback loop I guess. https://en.wikipedia.org/wiki/Elsagate I just found the Wikipedia article and I think it does a good job of giving facts about it.

Edit: What I mean by think it is a conspiracy is that you might find people who think it was some sort of orchestrated act to influence children. This was around the time of the pizza gate stuff for contex. I phrased that poorly. I meant people probably have conspiracy theories about the why part of it happening, but it was actually a thing that happened.