Most of the answers are actually in the core book, but they're buried in the bits of fiction and flavor write-up sections about the world: basically the Infernos are massive nanotech factories that scour the biomass of the earth and convert it into living weapons--the dreamseed.
The Engel are in fact the same children the grim riders tithe from communities throughout Europa. They are taken to underground facilities beneath Roma Aeterna and are converted into Engel, space marine style--muscle is grafted in, wings are bult on. The whole time the kids are drugged and brainwashed into forgetting their old lives and believing they are angels. The final part of the process is they are tattooed with a special nanotech ink that imbues them with potestae.
So long as they stay loyal they are fed mana from heaven--a foodstuff laced with drugs inhibiting physical development, instilling feelings of bliss and confidence, and making them suggestible. As soon as they are away for any length of time they start suffering withdrawl, which they are taught equates to losing their grace. They begin to mature physically and their wings go from white to whatever color their hair is--becoming dark Engel. Any engel who come to question the church are returned to "heaven" which is really a big incinerator in the lowest levels of the himmel.
There you go.