XXth century fascism was the west's capitalists reaction to communists taking power all around them. Rich assholes started financing rabid anti-communists to kill leftists.

An early version of fascism is described in Marx's 18th Brumaire, in which he describes a failed revolution being followed by a period of terror, when anyone the cops thought was involved with workers movement could be killed in the street.

From the end of WWII to now, imperialists have been financing far-right groups to counter communists/leftist/progressive groups wherever they seem to be getting some power. See operations Gladio and Condor, and also the Jakarta Method

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.

First and foremost, the Sturmabteilung was mainly formed out of the Freikorps, which was formed and used to crush the Spartakusbund and communist uprisings in 1918 Germany. The Nazis and Italian Fascist Party's first targets were always trade unions and the communist parties. The famous poem records the actual order of Nazi Priorities

First they came for the Communists

And I did not speak out

Because I was not a Communist

Then they came for the Socialists

And I did not speak out

Because I was not a Socialist

Then they came for the trade unionists

And I did not speak out

Because I was not a trade unionist

Then they came for the Jews

And I did not speak out

Because I was not a Jew

Then they came for me

And there was no one left

To speak out for me

As Moshe Postone elaborated in his analysis of Structural Anti-Semitism, that anti-semitism and using the Jew as a scapegoat allowed the separation of positive sides of modernity and capitalism (mass consumer goods, relative autonomy, etc.) from the negative aspects (alienation, volatility due to speculative finance, the parasitism of rent seeking, etc.) and say that the positive aspects were real capitalism, while the negative aspects were due to the Jewish controlled finance (a.k.a. cosmopolitanism, crony capitalism, woke-ism, etc.). Additionally, things like Judeo-Bolshevism allowed the Nazis to avoid questions like "who do we hate more, Jews or communists", and how they were opposed to finance capital and communism simultaneously.

But to be more direct, Fascism is fundamentally the white blood cells of capitalism. Where major cataclysms like the great depression, hyper-inflation, or the broader alienation of capitalist life prevents people from really believing in capitalism as this ever present progress that provides a "rising tide that lifts all boats". Fascism comes in to prevent any structural critiques or implication of capitalism by using scapegoats like the Jews controlling the banks or immigrants coming in and taking jobs. Fascism's fundamental material basis and mass support is based around people that don't want to give up on the idea of capitalism and their position in it (which is why the Petit Bourgeois is the most important basis of fascism and reaction) despite having to deal with the failures and consequences of capitalism by deflecting the blame onto socialism and some other group like the Jews. "The problem with the economy is that we still haven't privatized (Privatiserung) and deregulated enough of the economy. There is still some occluded socialism somewhere."