How can you say lot of difference when you are not even watching the Brazil game. I watched them and I know on the possession transition, Fred is aligned with Casemiro as midfield two and sometimes cover the left area if needed. During defensive transition, he shifted bit on the left to help the left back which he tends to do at United helping Shaw while in Brazil this allows Neymar stays up front.
High energy, brilliant in pressing, cover lot of ground, mobile, and likes to play progressive. He plays in Brazil and United because he offers those. If it wasn’t because this season performance at United, he wouldn’t be getting the call in Brazil to compete in copa america and World Cup qualify. players perform, they will be called to join Brazil national team. It’s not rocket science, if Fred doesn;t perform in the club then he won’t be called to join in the first place like in 2019.