Question: why is mjolnir in endgame?
Answer: The cherished weapon from Nidavellir that Thor held for years was destroyed in Thor: Ragnarok by Hela, but it makes a return in Avengers: Endgame thanks to the time travel plot. It is originally Thor who summons Mjolnir when he and Rocket go back to Asgard in the year 2013, during the events of Thor: The Dark World.