By the time the final act of Christopher Nolan's Oppenheimer begins, the movie takes a surprising turn. It's already detonated the first nuclear explosion test by Cillian Murphy's J. Robert Oppenheimer and his team. This quickly pushes the US army to drop bombs on their Japanese enemies. However, while the action dims down, there's a lot more drama to come in the final hour.
Source:https://www.cbr.com/oppenheimer-ending-explained/