Dominic Cummings said in his own words that he "hacked the referendum" by emphasizing the fears of some sections of the British population. He and his team deliberately created a "barrage" of false or misleading information and carefully directed it at British citizens online from personal information that was stolen or obtained in ways apparently unconnected to politics.
It was a form of "psychological operations" usually used by the secret service.
Everything is explained with factual evidence:
www.linkedin.com/pulse/how-dominic-cummings-hacked-eu-referendum-lloyd-hardy