Some definitions.
Alt right is just a rebranding effort, it doesn't have a cohesive ideology. But people within that sphere tend to be white nationalists/supremacists, misogynists, antisemites, "traditionalists", and bigots of various stripes. You can be a hardcore conservative Christian (evangelical, Catholic, Orthodox, doesntd really matter), neopagan, Hindu or whatever. They're not all fascists - many are more libertarian/ancap - but fascism is prominent.
Fascism itself is a broad, reactionary ideology that positions itself as a "radical centerc /third way, though most of everyone else would call it extremely far right. Mussolini was influenced by anarchism (paging
kristoffer) and specifically syndicalism, believing that the economy had to be restructured away from pure capitalist bourgeois control into a more "organic" system where people of all classes worked together under the guiding hand of the state for the good of the nation. They idolized action, violence, and technology.
Hitler went full-on crazy with it by slapping a racial element on top, making it all about Aryanism. The idea of the fascist variant of "national socialism" is thus to defend and propagate a particular race with the power of the state, yanking control of the economy away from "cosmopolitan" elites who are only in it for themselves and have no roots in the nation (for Hitler, this meant Jews; these days its "the swamp"/"globalists"/basically Jews). This is why Nazis consider themselves "socialists" while literally no other socialists do and, in fact, most socialists see fascism as a final protection of the bourgeoisie in response to the rising power of the left. I think you can see the parallels to that fairly easily in people like the Mercers.
