Why do Americans (and scholars in this sub) use fascism and Nazism basically as synonyms while here in Italy I've never met anybody doing it (included during my experience studying contemporary history at the Uni)?

Leaving aside the issue that it's not really in our brief to investigate the casual use of language from average people, this answer helps define what is fascism as separate from Nazism, which admittedly overlaps with the other types of authoritarian nationalism practiced in Europe during the 1930s/40s.