In terms of level of play and strokes, I don't see much difference between 2019 and 2021 Medvedev.
In 2019, he entered the match with no expectations, and once Nadal let him in the match, he was a tough opponent; Nadal was just clutch in the beggining of the last set.
In the 2021 AO final, I think Medvedev had high hopes, and got anxious. He wasn't that far away from Djokovic, who kept saving points, and Medvedev couldn't deal with that. Having beaten Djokovic in previous matches, he probably saw himself with more options than he felt he was having. Later in the match he just gave up, or so it seemed to me.
In the 2021 USO, he was calm and confident. Got no issues with his serve in the first set, and stayed clutch at the beggining of the second. In the third, Djokovic was playing more aggressive and you could tell that being close to winning, Medvedev got a bit tight. The game in the 3rd set to go from 4-5 games, he managed to win (confirmind the double break), but he felt the pressure of the moment, and of the audience, who were chearing more and more for Novak.
The biggest difference in all those 3 matches is the mindset, not the gamestyle, or level of play.