I'm a little confused about the nature of fae.

Progressing through Il Mheg we learn that Pixies are said to be born from the souls of dead children, and this explains their childlike behaviours.
Meanwhile the Fauth are said to be the souls of those who drowned, they're more malicious and seek to drown others.
Nu Mou are eager to please humans (or 'spoken races'), I haven't seen any overt lore ofr this yet in game, but I'm guessing they're... dogs?
And then the Amaro are living beings who have been imbued by fae magic to allow them to speak.

Presumably, they're all former Voeburtans, but where did the fae magic come from?
What made these souls manifest as fae creatures with such powerful magic?
Why aren't dead souls elsewhere capable of similar magic?