Sometime in the five years after Thanos snapped his fingers and unleashed The Decimation, Bruce Banner spent 18 months in the gamma lab and figured how to combine his intellect and personality with Hulk’s physique, and Smart Hulk was born… or Professor Hulk, if you prefer.
We were promised for a long time that the Hulk arc that stretched across Thor: Ragnarok, Avengers: Infinity War and Avengers: Endgame would have a big payoff, and that’s exactly what happened.