What is the cheese, lettuce, tomatoes, etc. in a sandwich called collectively? For example, I know that mushrooms, cheese, peperoni on a pizza are called toppings.
They would be toppings in AmE.
In BrE, a topping would not be covered by a second slice of bread- a filling would.
"Fillings" might be used with something totally enclosed, like a calzone. Even then, it wouldn't be weird to call them "toppings," since they are essentially the same choices as the toppings for a pizza. And you would normally buy a calzone from a pizza place.
It would appear to be another of those differences between the variants.
IMO, we do not say "filling" for sandwiches because "filling" is what goes in pies. And by pies, I mean fruit pies, not some weird meat-filled thing.