Question: how zamasu get goku's body?
Answer: He didn't switch with the Goku of Future Trunk's timeline, he switched with the body of the Goku in a variant of the main timeline where Goku hadn't died of the virus. It's confusing, so to sum it up, Zamasu got defeated by Goku, gathered all the Super Dragon Balls and switched bodies with Goku, becoming Goku Black.