First of all, there is no such thing as "real" femininity. Femininity by definition is "the quality of being female." So anyone who is female is by nature feminine.
What we usually call "feminine," though, which is basically nothing more than a cultural construct, it is indeed mostly just from cultural pressure.
There are some biological differences between boys and girls, mostly brought on by hormone exposure, which is generally what accounts for children's natural tendencies toward either being more quiet and social or more aggressive and competitive, more "alpha" or more "beta," but that's where it ends. The rest is added on by culture and socialization.
Culture takes those natural tendencies and completely throws them out of proportion. It teaches girls that they aren't supposed to be leaders, that leadership is unfeminine. It teaches them that math is unfeminine. They grow up with toys that are all about social skills rather than construction and engineering and math and science. And they're constantly taught that the number one thing that they need to value about themselves is their appearance.
In Elementary school, the amount of children who want to become president is equally 50% girls and 50% boys. By the time they reach middle school, though, that number drops to 20% girls and 80% boys. That should tell your right there the impact that socialization has on girls, and that the difference is social rather than genetic. In countries where the culture has been deemed to be the least gender-biased and least gender-segregated, namely Iceland, Sweden, Finland, and Norway, girls actually scored better than boys in math. In other countries, even though girls only scored 2% lower than boys on average in math, they were 40% more likely to say that they felt "helpless" while doing a math problem.
If you've never seen
this video, where a teacher started segregating students based on the color of their eyes, and those with blue eyes were told that they were smarter and superior to those with brown eyes, watch it. It shows the power that socialization has on us. After a while, these cultural constructs started becoming true. Self-esteem dropped in the group that was told they were not as good, and grades started dropping. And it was all just based on a social construct of being told how they weren't supposed to act, and what they weren't supposed to be good at.
Watch a few feminist videos, and you'll see pretty quickly where all of this cultural femininity comes from. It comes from constantly being talked over by men, constantly taught that being smart is unfeminine, constantly asked about their appearance but not about their accomplishments, constantly asked about whether they have kids or a boyfriend or not, and constantly inundated with advertisements about beauty products and unhealthy body images.
Basically, our cultural notion of femininity is in and of itself a construct. It's something that women themselves have learned through years of being forced into the subordinate role by society. Which is why so many of these "feminine" behaviors you're probably thinking of involve being closed-off... crossing your legs, keeping your arms in, automatically apologizing for everything, walking with your legs more together, using diminutive language like "I'm not sure, but...", being circuitous rather than asking for things directly ("do you want to..." instead of "could you please..."), and why their hand gestures and body motions tend to be more passive, more "I hope I'm not getting in anyone's way," and why women are more likely to be afraid that they're not good enough, only applying for promotions when they meet 100% of the qualifications, while men on average will apply for one when they meet only 60% of the qualifications. These are all learned behaviors, which come with the territory of constantly being pushed into the subordinate role and constantly having your abilities diminished and questioned by society. And this is likewise why "male" social behaviors are all about taking up space, being direct, being confident, and "aggressive" behaviors that are confrontational and signal to the other person "there's a man in this space, move over."
Kids do not start out life with these behaviors. They're learned.
Likewise, we as trans women generally start learning them the moment we start self-identifying as female, because from that point on we're trying to live up to society's standards of femininity. As we start seeing ourselves as female, we start internalizing how a woman is supposed to act, and we'll emulate it. And once we are female socially, and we actually are subjected to these same expectations and social norms that all women are, they'll sink in even faster. There can be some conscious process involved, but for the most part it just takes time and it just takes seeing oneself as female and wanting to fit in as female, and they'll come. The longer you've been full-time, and the more people see you as female, the more you'll find yourself becoming "feminine" without even noticing it.
Speaking from my limited experience being full-time, (and I was a very big guy pre-transition... large built, big boned, and 6'2" tall,) the way people treat me has COMPLETELY changed. When you're male socially, you're constantly subject to openly competitive behavior... people telling you that you should play sports, acting defensive around you, assuming that you don't want to be bothered, and immediately assuming that you're a threat. I HATED that. Where now as a girl, even though I'm still just as tall, it's almost like that same personal bubble ceased to exist. People are now automatically speaking to me in a pleasant tone, automatically assuming that I'm nice, they're quicker to forgive me, I'm constantly being asked about boyfriends and family instead of whether I sleep with girls and go on hi-LAR-ious drunken escapades, and all of that other masculine bulls***. Plus when people ask me if I played sports now, it's in a much softer tone, almost like they're telling me "you played sports? Aw, that's so cute," versus the old "yeah! You played sports, you big tough manly man you!" The difference in how people treat you when you're female versus male is just completely ridiculous. As a guy, you're automatically assumed to be tough and confrontational, as a girl you're automatically assumed to be cute and passive and naive.
