The fight can occur in any container, usually with no plants etc... The fishes are added and they will....fight.
These fights usually dont last very long, one of the fighters will give up.
Truth
The truth is most bettas males dont die after fights. Well in the ring. Some may die due to their wounds if they are not treated by the owner. so the reason why people may say they fight to the death is either they are marons or the fishes die due to battle wounds which arent treated.