Devils come about in one of three ways, the first is simple, it is possible for one or two devil parents to produce a devil child. If a devil born to human parents and a wode elf had a child, there are two possible ways for that child to turn out, either they are a devil or they are a half-elf. This situation has devil some strain in relationships where the person giving birth is not a devil, they end up assuming their partner has cheated on them, especially in cases where the devil was born to devils who have lost track of where their ancestry originated.
The second possible option for a devil child is through a parent having a connection with infernal magic, whether that be a magical patron relationship, the person is simply a demonologist, or it could be that they gave birth while in the Hells. Somewhere, at some time, their child was exposed to fiendish magic that changed their physiology.
The third, and final, way produces devils with the most potent connection to the Hells, and that is through infernal conception. Most of the time, this conception is entirely magical, the greater devil magically replacing a child while still gestating in the parent's womb, though there have been cases where the devil was conceived through more mundane methods.