So from what I understand, the Lawful Evil Devils primarily perpetuate their race by making deals with mortals in exchange for their souls. A mortal who accepts this deal will rise as a lemure in the Nine Hells upon their death (or sometimes a more powerful kind of devil, depending on the deal). Devils also offer these deals to souls that have reached the Fugue Plane upon their death. A soul that takes advantage of such a deal is likely trying to avoid punishment in the afterlife and thinks damnation is better for some reason. That's all well and good.
Chaotic Evil Demons stage raids on the Fugue Plane, capturing souls that Kelemvor's servants can't save or just straight up ripping some souls off the Wall of the Faithless. Those souls become manes in the Abyss. I can't find any mention of a different way demons increase their ranks, but I may be missing some information.
So how do the Neutral Evil Yugoloths make more Yugoloths? There's no mention I can find anywhere of them interacting with the Fugue Plane or the afterlife in any way. Where Devils embody tyranny and structure (deals), and Demons embrace anarchy and wanton destruction (raids), Yugoloths represent selfishness and sociopathy, and it seems unexplained how they come into being.