In several of the psot-game guides for Pokemon Sword & Shield, I see that people say that you can rematch the gym trainers in the Wyndom stadium. They say that you need to invite them, but no one seems to describe how to invite them. They are not in their original gyms and they are not in Wyndom stadium.
I feel like it has to be something obvious since no one describes how to do it, but I just can't seem to see it.
Apparently you can also rematch Hop, but again, how?