Fascism is a reaction to leftist power in any given country it arises in. That’s why we call them reactionaries.
Fascism occurs when leftist growth in a country grows to the point at which it threatens to overthrow the bourgeoisie’s ruling class power. When this threat arises the bourgeoisie fund ultranationalist elements within nations in order to build a force that can be used for ultra-violence in the pursuit of killing off the leftist threat.
Fascism differs depending on national character. Fascism in Germany was not the same as fascism in Italy or fascism in Spain, Chile, Japan or India. But it often has certain characteristics of ultranationalism and the supremacy of certain groups, but not always. Ultimate fascism is anything that it has to be in order to get power and use ultraviolence which is why describing it in absolute terms is difficult. This is why fascism is more aptly characterised by what it is in reaction to rather than anything else.
In short. Fascism is like the white blood cell of capitalism, it arises when the “infection” of socialists threatens to overthrow it.
When the “infection” is over, it then ends, morphing back into regular capitalism. We can see that this occurs by looking at the countries where fascism was not defeated, where fascism won. In both Spain and Chile fascism did not become something unique from capitalism, it is a part of capitalism, and when it had eradicated the threat of socialism it was then changed by its ruling class back into neoliberalism, which is a more efficient means of wealth extraction from the population.