>>519554961 (OP)
Internet.
We no longer observe world by our own senses, we mostly observe it via media.
You select the media you want to watch most likely by how much do the facts they present agree with what you would like the world to be like.
Modern internet has very powerful algorithms that both suck you into watching more content and supporting your worldview, so you can surround yourself into a hugbox detached from reality and believe whatever the fuck you want.
It was also like that in old totalitarian regimes of 20th century, but now we achieved a point where you dont need authoritarian control over people to brainwash them, they brainwash themselves as the brainwashing has merged with entertainment they voluntarily consume multiple hours a day.