The Holocaust is one of those things that's just simply not up for debate: it happened just about exactly as people said it did

We have physical proof from the camps themselves - with the Soviets being the first to liberate concentration camps, and being the first to begin preservation of evidence

We have documentation from the camps, actual papers telling us exactly what they were doing. We have bodies, tattoos on survivors, empty Zyklon canisters. We have the actual minutes of the meeting at Wansee where the cold calculus of the Holocaust was laid out

While the aftermath of it has occasionally been politicized, usually around Israel, the simple fact of it existing isn't political. There's a good blog with an unfortunate name called Holocaust Controversies that debunks a lot of Holocaust denial and will provide primary sources frequently to do so