That sounds like it’s mostly random and it says later they just start changing with time until they become a unique being like a demon lord.
For devils it explicitly doesn’t matter.
According to that one thing that I don't really understand. The most recent lore is that Devils can't have kids at all expect with mortals. And I prefer it that way myself. Fiends don't have to have anything like normal biology that makes sense.
Erinyes are noted as the sole exception in Faces of Evil.
But Erinyes are presented very differently in 1e and 2e as they are more Devil versions of Succubi rather than their more modern Fury incarnation.
2e Faces of evil says Demons can’t breed with each other cause female demons are infertile.
Edit: Sorry mixed it up, demons can breed according to that book, devils can’t.
It depends on how the adventures are. Nothing in the set at all glance looks like it’s changed from the core game other than simpler character creation.