No it isn't. You're suffering from a form of apophenia.

> How do they do this?

This isn't happening.

> How is the data obtained?

The web is a big plac, but you're spending too much time online andhaving repeat interactions with the same media. You need to spend time away from screens and with other people IRL.

> Is there a way to block it?

YES! Never visit 4chan again and spend way less time browsing aimlessly online, and not only will this problem stop - but many other issues in your life will likely resolve.

Very likely the same way that targeted advertising is carried out, which most certainly does occur. Also, 4chan has a history with law enforcement, including the FBI, and the military. Moreover multiple studies have been carried out by academia including by the Israeli military in regard to demographics. The point is that an entity like Palantir could be applied to reduce hate speech and violence. Rather than serve an ad it uses AI to generate a post that is humorous yet very curated in an intrusive insidious fashion.