In Italy, at “L'Isola della Pizza” in Rome, I asked the guy if I could get a pizza with salami, pepperoni, and sausage, and the guy was like “ah, American style!”
The honest answer is this: Salami (sliced salami), pepperoni (sliced spicy salami), and sausage (pre-cooked fennel-flavored uncased/crumbled pork sausage).
In the US, “sausage” tends to generically refer to uncured, fresh, or raw sausages, often really meaning “ground meat mixed with herbs and spices sometimes in a tube or casing (but not always).”
Is it like the Italian American "shrimp scampi" where it's just the words for shrimp in two different languages? My understanding is that "salami" is just the Italian word for cured sausage.
Also, "pepperoni" is an Italian American word for a spicy salami that contains peppers, so it's just a type.
So, this was the only place that I personally found in Italy that even tolerated topping options for pizza, and I think they kind of went out of their way to accommodate Americans. Typically they have a limited selection of popular topping combinations; Margherita, bufala, etc., and you have to choose from that set. I think the place was unique in that way and they were pretty friendly about it.