who judges what sites to censor, key words or human eyes?

I don't know what you mean by "which sites". Most websites, like Yahoo Answers, are owned by private corporations. They have policies- as you will find with any employer, for example. Those policies may prohibit certain things- as your employer may prohibit the wearing of a bathing suit to work. That has nothing to do with the first amendment, which is about not being prosecuted by the government.

The one exception has to do with child pornography, which is a crime in every state, and by federal law. Those laws can supersede any organization's policies.

But if you're talking about Facebook, Instagram, Yahoo, etc., they're responding to public pressure to limit access to groups that are deliberately lying to manipulate the public. 

And yes, there are some algorithms that look for key words, but there are also human eyes.