The biggest music act on YouTube last year wasn’t Billie Eilish, Drake or Taylor Swift, but a 20-year-old Southern rapper who goes by the name YoungBoy Never Broke Again.

YoungBoy has posted 11 music videos that have each racked up more than 100 million YouTube views in the past two years. Born Kentrell DeSean Gaulden in Baton Rouge, La., he has built an ardent following with a reflective and melancholic style of rap. He releases material at a frenetic pace, and often posts it on YouTube first—by himself. His music videos,...