At least, that's how Earth invasion was described in an old horror show, one made a few years before the true invasion happened, ironically enough.
The truth is that god forsake us, condemning not only humanity, but all form of life on Earth to a slow and painful death. But since it would also do the same to demons, Hell's armies intervened. Let's be honest, they didn't do it by compassion, and weren't really gentle about it.
First, you need to know that demons can't go to Earth. Or rather, they can, but would be all ghost like and wouldn't be able to interact with the world. There is a "work around" of course, they need to posses something that live on Earth, humans being what is the most comfortable for them.
So when Earth invasion really happened, everyone saw peoples transforming into all kind of monsters around them, sometime even friends or family members, without apparent rhyme or reasons. Some of those monsters being more or less humanoids in nature, some going so far as to look like the classic representations of Imps, Succubus/Incubus, or devils. The others ranged from creepy as hell to downright nightmarish, and not all of them could be described.
It didn't take long before Hell annexed Earth. Once it was clear we had lost, the demons explained what happened. Of course very few believed them.
…Until God himself declared it the truth and that, since the demons wanted Earth so much, they could have it.
So Hell's armies took control of every governments, the ability to govern ourselves striped from humanity.
Of course most demons didn't want to stay 24/7 on Earth and wanted to go back to their loved ones, even demons have kids and parents after all. So they put in place a planing on who would go back to Hell when, so there was always of them around to stop humans from rebelling against their new overlords.
But it revealed a new problem: While demons could posses any humans, only part of the population could be safely unpossessed, the rest dying upon being freed.
Even today, billions of years later, nobody understand why or how. All we known is that, upon starting puberty, some human kids gain the ability to be safely possessed, without any detrimental effect on their life beside loosing control of their body for the time of the possession.
…Along with potentially taking the appearance of the demon possessing them. (Only for how long said possession is maintained, again.)
So nowadays, when a special blood test reveal a young one can be possessed safely, he is send in to Hell in a demonic school, where he learn how to be a good host for the demons who will possess him once adult, along with whatever subject he fancy.
There is typically two way parents react when they learn their kid can be safely possessed. They either cry from sadness, knowing they won't see much of their child anymore, or outright reject him, like it was his fault. (Spoiler alert: it isn't.)
Once send to those school, they will go to class not only with others like them, but also demonic kids. While the kids of the two species usually don't mix, friendship between them isn't unheard.
And today, once adult, most gendered demons prefer to use a host of the same gender. Said host is more comfortable, and modern demons think it a natural courtesy to make thing the most comfortable they can for their host, they do lend them their body after all! (Even if they don't have a choice in the matter.) But being possessed by a demon of the opposite gender still happen from time to time.
Only one question rest: what kind of people will be the central protagonist of our story?