Steve chose Sam to be the new Captain America, but when Sam sets the shield, and the identity, aside because he doesn't feel like him taking over the role honors it, the American government chooses a new Captain America. Now Sam and Bucky have to deal with the consequences.
Although Loki died at the beginning of Infinity War, another version of him broke free of the timeline mid-way into Endgame. Now that Loki is back and he's gained the ire of the Time Variance Authority. He's fighting against the constraints of the timeline for the fate of free will.
Slotting in between movies two and three, the "Clone Wars Epic" is really three different piece of the series tying things together to tell an overarching story of the greatest war the Republic ever saw.