They would stay in the little “cups.” Bettas aren’t terribly successful in larger tanks.
I know they are aggressive, but I do recall years ago they had tanks with dividers so the fish had room to swim but no contact. Well I was never a fish person though I enjoy watching them I still don’t like animals treated badly.
We tried putting ours (just one) in a 15 gallon tank, maybe? It sat under a rock the whole time it was in the tank, lost his fins. Put him back in his little “cup” home and he recovered beautifully.
The used to not have holes for the fish to get air exchange. They have finally changed that.