How does becoming a demon or an angel work? Like being turned into one?
Angels have it simple. To become an angel, someone who is kind and led a good life will always become an angel when they die. Pures will always become angels when they die. Humans can be chosen early on to become angels. White-lighters are the only species who have the power to change humans. With a simple kiss to the lips, white-lighters can give anyone wings. A turned angel, or any angel who becomes one after death can become any of the species except Uni's, and Atiel's. Those two angelic species can only be born naturally.
Angels can also lose their wings. This is called ‘Falling From Grace’. If an angel loses their wings they can do simple white-lighter tasks to gain them back again.
As for demons they have more ways.
When a bad person dies, they do not become a demon, they are sent straight to limbo. But a contractor can summon their souls back from limbo, and though their souls are indebted to the contractor, they can live life as a demon.
Pures can become demons in different ways. If a pure has sex with an incubus by their own free will, without demonic influence, they will become Impures because of their decision. They can also become a demon just like any other human or angel could in the following ways;
Flesh Eater Demons: When a flesh eater’s (which is most of the demonic population) blood gets into a human, angel, or Pures bloodstream that person well become a demon.
Incubi: Incubi can change another into an Incubi by biting and breaking the skin of their victim. Incubi have a close relation to snakes. There is a venom in their teeth that changes Pures, angel’s, and humans into Incubi. It has no effect on other full demons.
Mutts: Similar to Incubi, mutts have sort of an infectious causality to their teeth, but also their nails. Mutts usually kill their pure, human, and angelic prey. But if a demon were to get scratched or bitten, they would slowly become tainted, and turn into a mindless mutt. Mutts are also created whenever a human who doesn’t have what it takes to be a demon, gets turned into one.
Vampires: Vampires are made whenever one drains their victim of their blood. Because of the law to never fully drain a human, it doesn’t happen very often.
Ilahi, Harpies, Imps, and other species can only be born, never created.