It all comes from Mein Kampf. Either they've read it, or they're repeating someone else who has read it and changed some of the language slightly. Let me try to explain.
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During WW1, Jews in Germany were fighting for the German Realm, but were still allowed to practice their Judaism, which meant not attending or donating to the Catholic Church. They were portrayed as greedy and weak and blamed for Germany losing. This, along with German propoganda films like The Golem, is where a lot of these Jewish stereotypes came to be, like how black people were portrayed in The Birth of a Nation. This is only the surface, however.
The book, Mein Kampf states that African soldiers from French colonies had destroyed the Rhineland and impregnated pure white German people. Germany didn't have any black people before WW1, and their few African colonies were taken away completely in the Treaty of Versailes, see https://en.wikipedia.org/wiki/German_colonization_of_Africa for further references. The Treaty of Versailles also removed part of the Rhineland to France, which France then militarised, and Germany couldn't, and East Prussia was separated from the rest of the country with the newly formed Poland. British economist and Versailles negotiator John Maynard Keynes summarized this position as attempting to
"set the clock back and undo what, since 1870, the progress of Germany had accomplished."
Keynes, John Maynard (1920). The Economic Consequences of the Peace. Harcourt Brace and Howe.
Side-note: Keynes was also known as anti-semitic and Germany did really successful after adopting Keynesian economics.
More importantly, the Duchy of Warsaw and the Confederation of the Rhine were created during the Napoleonic wars, which were apparently funded by the Rothschilds (Jews) and Germany had mass unemployment and inflation after the treaty and of course, Hitler blamed the Treaty, the depression and, as mentioned before, race-mixing on Jews.
One final point about WW1, is the Balflour Declaration. It promised a land for the Jews in Palestine, then under Ottoman rule. This was to gain Jewish support after the Russians had dropped out of the war. It's no surprise that a pure German soldier in WW1, who later joined the Socialist Worker's Party, would hate Jews given these facts.
Mein Kampf talks about the evils of "Jewish migration," "the Jews attempt of race-mixing to destroy white genes and keep white people inferior/slaves to Jews," "the threat of Jewish supremacy/elites," and constructs it as a struggle for the German workers. In context, anti-migration, race-mixing, unemployment, elites and how it's a struggle for the common man, or the workers is all taken from Mein Kampf. I believe the reason holocaust deniers are viewed as an infestation of the right wing and not the exact same people is because a lot of right wingers try to pretend they're free market capitalist (Keynes) or libertarian (Jefferson) or Jewish themselves so condemn things like Hitler and Socialism and even equate the two like Ben Shapiro.
Side-note: Shapiro kind of has a point because National Socialism is a very concentrated part of Socialism, but only for a very particular kind of race like they have in North Korea. It's nowhere near the philosophies of Marx or Lenin.
Make no mistake, you could listen to all these alt-righters or even a Trump speech and it sounds like something out of Mein Kampf but without the anti-semitism. Even more ironic, is Arabic countries who are unashamedly anti-semitic, believe the same things about Israel.
It's the same shit we've had for over a thousand years.