A great, short book about this is Blackshirts and Reds by Michael Parenti. Or if you’d prefer a short pamphlet about what Fascists are all about and how they came to power in Italy and Germany, there’s a collection of writings by Leon Trotsky called Fascism: What it is and how to fight it. Reading up on how fascism was further funded and used around the world by the United States after WW2 to fight communism is also a great idea, there’s the Jakarta Method by Vincent Bevins which gets into this somewhat and generally researching things like the Operation Paperclip and Klaus Barbie and the rehabilitation of former Nazis in NATO etc.

But a short version: fascism is fundamentally a violent reaction against socialism and communism (movements which seek to take power for the working class). It’s footsoldiers tend to be of the petit-bourgeoisie, armed and organized by the Big Bourgeoisie (capitalists) to violently destroy any opposition to capitalism when capitalism inevitably fails to meet the needs of a functional society. The Nazis came to power by promising to destroy communism in Germany, obtaining funding and support from domestic and international capitalists to do so, and making good on that promise by using armed force to destroy leftist movements in the streets and eventually taking state power and using that state power to unabashedly persecute and kill communists and leftists. The first concentration camps were opened to hold communists, socialists and trade unionists.