Fascism is a form of radical authoritarian ultranationalism, characterized by dictatorial power, forcible suppression of opposition and control of industry and commerce.

What exactly is the difference between this and stalinism? How can anyone look at the definition of fascism, and the definition of communism, and them swear up and down that stalinism is communism and not fascism?

Communism:

the common ownership of the means of production and the absence of social classes, money and the state.

Stalinism: a ruling social class owns the means of production. Dictator. Money. State. Death squads for dissenters.

Stalinism is fascism. Not 'red fascism'; just straight up unapologetic fascism.