in this story the world of jujutsu kaizen is the past of my hero academia and cursed techniques have evolved into quirks. occasionally a person is born with something called a legacy quirk. it's the evolution of a cursed technique from the past. when someone is born with a legacy quirk they could still have their own quirk as well as the legacy quirk or they could have their own quirk replaced by the legacy quirk. legacy quirks can be the technique of either a person or a curse. domain expansions don't exist here