“There’s one way this is going to go,” he told the man. “You’re going to end up knocked down unconscious.”
Over the more than two years he helped produce and publish videos for Mr. Robinson and others, Mr. Robertson learned how making clever edits and focusing on confrontation could help draw millions of views on YouTube and other services. He also learned how YouTube’s recommendation algorithm often nudged people toward extreme videos.
“It meant that we did more and more extreme videos,” Mr. Robertson said.
Caolan Robertson grew up in Ireland, and after his parents divorced, he moved with his father to a predominantly working-class area in the north of England. Realizing from a very young age that he is gay, he often felt like an outsider. But he says he encountered more overt homophobia when he moved to London for college and walked through the largely Muslim neighborhoods at the East End of the city.
After the 2016 shooting at a gay nightclub in Orlando, Fla. — where a Muslim man pledging loyalty to the Islamic State killed 49 people and wounded 53 more — Mr. Robertson developed an extreme animosity toward Muslims, particularly immigrants. His anger was fueled in large part, he said, by videos he watched on YouTube.
He began watching videos from mainstream outlets, like an episode of the HBO show “Real Time With Bill Maher” in which Sam Harris, an author and a podcast host, advocated greater criticism of Muslim beliefs. YouTube’s recommendation algorithm suggested more extreme videos involving personalities like Mr. Robinson, a former member of the neo-fascist and white nationalist British National Party who was born Stephen Yaxley-Lennon.
In 2017, Mr. Robertson contacted Mr. Robinson and soon began working with him as a video producer. By the end of the year, he was also collaborating with Ms. Southern, an activist from Canada.
Knowing what garnered the most attention on YouTube, Mr. Robertson said, he and Ms. Southern would devise public appearances meant to generate conflict. That December, they attended a women’s march in London and, with Ms. Southern playing the part of a television reporter, approached each woman with the same four-word question: “Women’s rights or Islam?”