A question about the father machine and void- YNP

So I understand how the father machine is running off the jump started souls of the townspeople .

He has a Command- Stack rocks z attract spirits for energy, to stack more rocks to attract spirits for energy ect.

I am confused about the connection between the miasma that is discarded when he removed a person's Soul- their inate Investature. I assume that is their Identity but WHY would that take the form of a black void and why are they now labeled as Cognitive Shadows? The townspeople have no extra Investiture keeping them here.

I do see the Hion Lines connecting to Yumi, so is that the Connection that's keeping them here and their control by the father machine? It's pretty vague.

I have to go back and reread but I didn't see anything that said Why the Nightmares attack and "eat" people. Is this stealing souls and giving it to the father machine as well?

Nightblood also is similar to FM in where he eats whatever Investature is in you but there is no Connection formed there and no black Void must created from the discarded. .whatever from the remains. So what's the difference?