I was around those types of communities but never really into them. Why do they make girls want to be yaoi boys?
From a girl who observed them...
The classic answer is that Yaoi attracts young girls who want to feel like equals with their romantic partner by having two of the same people with the same physical playing field. If you ever try to crawl through the cesspit of YA literature (please don't, it will make you want to claw your eyes out regardless of gender), there's still a bunch of fucking weird romance tropes that have women getting the short stick to a more powerful man. The Alpha Beta Omega werewolf hierarchy, the arranged marriage tropes (surprisingly still popular), and other thing where men act dominant towards chicks to almost the point of being rapey. Young Adult really aims at 20s-30s year old nowadays but it is still promoted to teens and tweens, & there are more not awful stuff too, but this other shit is prevalent enough to be worrisome. Anyway, all that negative shit makes young teen girls first exploring their sexuality shy away from being an object and closer to someone who's allowed more power in the plot, the man. Two men is an equal playing field in their mind. Most fanfics written by younger or yaoi brained women just have one man as basically a woman (self insert), and the other as a man written by a woman (the man/pitcher/etc of the relationship). The gayngst also lets them agonize on their own budding feelings of sexuality & lets them curse love and sexual feelings while getting into it.
Having only men also helps them space themselves from their own bodies. Not many girls like getting boobs or periods, and basicslly NO girl likes being a tasty piece of meat to old perverted men (no one does). Having 2 men instead lets them see TWO hot guys at once as well as not see themselves (as teens have body issues) or what they feel like other people creep on them for. "Succulent breasts" is funny to adults but to teen girls just developing into puberty the phrase is fucking disturbing.
Teens who fall into this part and don't grow out of it can go into female to bishie territory, where you don't want your body to be preyed on, you don't want the masculine traits of the people who prey on you, but you still want to beautiful, so go for the object of romantic purity.
BUT the reasons are more diverse than that. Women have their own version of auto-erotic target displacement where they spent SO much time cooming to yaoi boiz that their brain becomes fucking weird and they want to be the yaoi coomers. This is prevalent in the autism umbrella of girls, as autism tends to give people even weirder fetishes, but can be spread further out to more non autists because there are some women who still have the capacity to develop fetishes. There are a legitimate number of Aidens who transitioned to become such, and the best reason as to why is to jerk off.
There's also girls who just want to belong. Sarah's friends with John, Becky, and Jasmine. John has come out as gay either before or after the girls met him, whatever, but there's
usually at least one LGB in there, sometimes also fake but that's besides the point. One day, Becky turns to Beckett, everyone praises Beckett. Then Jasmine becomes Kit, and everyone claps. Everyone talks about how shitty cis and or hetero people are. Sarah, not wanting to be excluded for being either, turns nonbinary or to a guy so she doesn't get dumped by her group or have underlying hatred with them. She doesn't go full man, she goes to a pretty boy so she can have fun with fashion instead. Which is funny, because Becky did it so she could be a trailblazer and get praise and attention, while Jasmine did it for rebellion against her parents reasons - both of them are going "soft boy" or "yaoi man" or "kpop" because of it too, they don't want commitment but want to look really cool masculine (and cool masc is who they're attracted to, not action film buff guy or lumberjack woodsman). More can spread this way too if they have a larger friend circle.
Hell, plenty of nerdy cosplayers tend to go nonbinary and transmasc just because they like playing dressup and don't want to be cis because it doesn't feel as meaningful to them. Cosplayers just love dress up and acting, give those bitches (woman or man) five seconds to be someone not a pasty lame nerd from bumfuck nowhere and they'll jump on it.
Another note: body disconnect is common with people who have anxiety, adhd, autism, and other disorders that give dissassociation. It isn't just a word tumblr and twitter beat to the ground, it is a legitimate medical symptom where you don't feel in tune with your body. So if you're one of those types of people and feel more like a little man in the brain piloting a big meat body, then you're going to find transitioning appealing since it seems like an answer to possibly connecting to the body or at least making yourself ideal (e.g. Kpop boy pretty or androgynous in general). Sometimes people just transition in false belief that it will cure another symptom.
Lastly: have you ever met someone or observed a cow with Borderline Personality Disorder? You likely have even if they were not formally diagnosed with it, see
the symptoms here. They're a tempest of a person, with one of the key factors being a need to find an identity. Becoming a yaoi boi gives them a pretty charismatic stereotype to try and live up to, something that attract people and praise to you(in theory, not when they try it).
Are there women who troon out who don't want to be yaoi boys? Totally, some do want to be buff guys, leather daddies, or even hairy fat men (I'm not fucking kidding, I just don't have examples on hand, try the sjw art thread). They troon for the same reasons I outlined above. Younger women just prefer more androgynous, sensitive looking men in general, which can change over time.
Also lastly: it's easier to become androgynous than super masculine, especially if you're Ethan Ralph height. It's more obtainable before you go on Testosterone, IF you go on Testosterone & aren't doing it as a fashion thing. Old age helps make fools of us all, but that will really kick you in the crotch until you hit the age of old where everyone looks similar again.