You're referring, I presume, to the purported "backstory" of how humans discovered soap, most popularly described in the 1999 film "Fight Club" (which I should hope it goes without saying isn't a documentary, but I'm sure the story has been repeated elsewhere and may predate the book the film is based on). To quote Tyler Durden, a very unreliable narrator to be sure:

Ancient peoples found their clothes got cleaner if they washed them at a certain spot in the river. Why? Because, human sacrifices were once made on the hills above this river. Year after year, bodies burnt. Rain fell. Water seeped through the wood ashes to become lye. The lye combined with the melted fat of the bodies, till a thick white soapy discharge crept into the river.

That's a nice story, but it's not historical. We do have archaeological evidence that the process of soapmaking (mixing fat with ashes) was known to the Babylonians circa 2800 BCE. In 1550 BCE, the Egyptians documented making soap with animal fats & alkali salts. But given that these are historical examples of people documenting an industry/discovery that was already know, they don't tell us how soap was "discovered." There's no record somewhere of some early, early proto-scientist who said "Hey, I figured out why the water by the sacrifice place gets our whites whiter" even if (and it's a pretty big if) that was how the chemical process of saponification became known to humanity.

The soap story as outlined in Fight Club & other sources is ahistorical wish fulfillment/assumption, almost like how backronyms develop to explain already existing words.

This is a bit astray from your general question, but since you mentioned the Holocaust, it bears mentioning that obviously the Shoah in the 1940s had nothing to do with the invention of soap, however, the Nazis are now intrinsically linked to the concept of soap made from human corpses. While the Nazis did experiment with making soap, they never engaged in the practice on an industrial level. The idea appears to have spread mostly through propaganda used by the Nazis to further terrorize camp inmates, according to Yad Vashem. In fact, the trope of "the Germans are making people into soap" dates back to WWI, though at that time, it was their own war dead they were accused of using. Hope this helps.