As a teenager in rural China, Zeng Jiajun used his internet know-how to watch a banned documentary on the bloody military crackdown in Tiananmen Square. A decade later, he was part of China's sprawling censorship machine. After graduation, he joined ByteDance, an upstart Chinese social media company that owns TikTok, and worked as part of a team that developed automated systems to filter content the company did not want on its platform.