Li Chengzhi had a lot to learn when he first got a job as a professional censor.

Like many young people in China, the 24-year-old recent college graduate knew little about the 1989 Tiananmen Square crackdown. He had never heard of China’s most famous dissident, Liu Xiaobo, the Nobel Peace Prize laureate who died in custody two years ago.

Now, after training, he knows what to look for — and what to block. He spends his hours scanning online content on behalf of Chinese media companies looking for anything that will provoke the government’s wrath. He knows how to spot code words that obliquely refer to Chinese leaders and scandals, or the memes that touch on subjects the Chinese government doesn’t want people to read about.

Mr. Li, who still has traces of youthful acne on his face, takes his job seriously. “It helps cleanse the online environment,” he said.

For Chinese companies, staying on the safe side of government censors is a matter of life and death. Adding to the burden, the authorities demand that companies censor themselves, spurring them to hire thousands of people to police content.