Note: Transsexulism is a better word for the title, however I can't edit that in.
I feel, personally, a good place to start in this subverse is to start with a general explanation of transsexualism. Be this for people questioning their trans status, or a cisgender person genuinely curious about Transsexualism.
First and foremost, Transsexualism and Transgenderism are both the same. That being said, transsexualism became less used over the years due to it's close ties to pornography. Both are typically fine to use, though some may find offense in the use of the term transsexual. This being said, being transgender or transsexual is not the same as being a transvestite or crossdressing. Typically these people dress up as the opposite gender for a show, sexual pleasure, or just because it feels nice to them. Not all crossdressers are transgender, but some transgender people may crossdress (usually for moneys sake, but there are other reasons.)
Transsexualism goes a bit deeper than that. Being trans is when an individual has the opposite neurological sex. From the studies we've acquired over the years, the reason one does this is based on the fact that what is known as their "brain sex" doesn't match up with their birth sex. This causes intense dysphoria in an individual in regards to their sex characteristics. Though these aren't the same, think of the distress someone suffering with anorexia may feel when they look in the mirror. They may be skinny, but all they see is fat, it causes them distress and may end up in some form of self harm (such as starvation). Transsexualism works a bit differently. A transsexual will expect to see the body of the opposite sex, or seeing their body causes them intense distress as it doesn't look how it should. Some trans people also talk about experiencing phantom limbs, and even dissociation.
So why does this happen?
We mainly have theories, but to list a few;
"The androgen receptor (AR), also known as NR3C4, is activated by the binding of testosterone or dihydrotestosterone, where it plays a critical role in the forming of primary and secondary male sex characteristics. Hare et al. found that male-to-female transsexuals were found to have longer repetitions of the gene, which reduced its effectiveness at binding testosterone" Source
*"A variant genotype for a gene called CYP17, which acts on the sex hormones pregnenolone and progesterone, has been found to be linked to female-to-male transsexualism but not MTF transsexualism. Most notably, the FTM subjects not only had the variant genotype more frequently, but had an allele distribution equivalent to male controls, unlike the female controls. The paper concluded that the loss of a female-specific CYP17 T -34C allele distribution pattern is associated with FtM transsexualism" Source
*"In a first-of-its-kind study, Zhou et al. (1995) found that in a region of the brain called the bed nucleus of the stria terminalis (BSTc), a region known for sex and anxiety responses, MTF transsexuals have a female-normal size while FTM transsexuals have a male-normal size. While the transsexuals studied had taken hormones, this was accounted for by including non-transsexual male and female controls who, for a variety of medical reasons, had experienced hormone reversal. The controls still retained sizes typical for their gender. No relationship to sexual orientation was found.
In a follow-up study, Kruijver et al. (2000) looked at the number of neurons in BSTc instead of volumes. They found the same results as Zhou et al. (1995), but with even more dramatic differences. One MTF subject who had never gone on hormones was also included, and who matched up with the female neuron counts nonetheless."
We've also caught matter differences on scans, in which someone who has the opposite neurological sex, aligned with the opposite sex as opposed to the same sex Source
And of course there's also the famous David Reimer Case Rest In Peace.
Through this, we can conclude that transsexualism is rooted heavily in the brain and not due to environmental factors.
Transsexual Terms, and their meanings;
DFAB - Designated Female At Birth. A word used for an individual who was born and designated to be the female sex.
DMAB - Designated Male A Birth. A word used for an individual who was born and designated to be the male sex
FtM - Female - To - Male. Someone dfab who transitions into male
MtF - Male - To - Female. Someone dmab who transitions into female.
Passing - Passing as the gender you are. E.g. someone who's FtM passing as male, being referred to by male pronouns, etc, by others who don't know them.
Non-dysphoric trans person - A cis person.
Will add more if requested.
General Trans FAQ:
"If transsexualism is biological, why do I need to refer to a trans person with the right pronouns?"
First and foremost, pronouns are a reminder of things that a lot of trans people just don't wanna think about. I don't wanna be reminded of how badly I pass, or the fact that I wasn't born right, and overall it's just nice and courteous. It can trigger dysphoria in some cases as well. On top of this, if someone is actually passing, you don't want to be the person who notified Johnny-fucks-his-cousin that there's a trans person nearby. Outing a passing trans person can be very dangerous. Be courteous, it's not that hard to bring out the word he instead of she.
"Why do transgender people get mad when I ask about surgeries?"
It's another common courtesy thing. It's not exactly kind to ask about whether someone still has tits or not, or what genitals they have. I don't go around asking Jewish men the state of their penis, I don't question women with breast cancer about whether they're gonna cut their tits off, there's no need for you to know a trans persons surgery state unless you're gonna be having sex, and even that only extends so far. If a trans person is willing to share their medical experience with you, awesome! But if not, google does have a range of answers for you about surgeries if you're honestly that interested.
"I still don't understand transsexualism"
You don't have to understand. You know it's biological, you know it can't be helped. You don't have to intricately understand the ins and outs and whys. So just be supportive. I don't understand a lot of medical conditions, but I'd still be courteous to those with them.
"I think I'm trans, how can I tell?"
Only you know how you feel, no one can really tell you you're trans. My best suggestion is to think of why you think you're trans. Ignore gender roles and gender expectations, even ignore pronouns. Think as though you were in a completely non-gendered society, would you still desire to change your sex characteristics? Would you feel uncomfortable with your sex characteristics? There's no rush in figuring out whether you're trans or not. Don't disregard whether you're trans based on age, sexuality, skin color, whathaveyou. I've often heard of trans people disregarding whether they're really trans because they're not the "typical" sexuality or they're "too old" and should've noticed earlier. Some people take a long time to really hit it. And being trans has little to do with anything but how you feel in regards to your sex characteristics.
"Instead of transitioning, why don't we just focus on making trans people comfortable with their birth sex?"
Because that's conversion therapy, and it's abusive. You can't change someone's transsexual status. You can make them hide it, or make them feel unwanted, and that'd be fine if you're okay with trans people committing suicide at an even higher rate. But that's not right.
"But trans people commit suicide more after medical intervention! Obviously medical intervention doesn't work!"
There's no link in any study that trans people have committed suicide because medical intervention doesn't work. The more likely reason for committing suicide is a lack of acceptance in society. Being kicked out of home or disowned by the family, ignored by your friend group, being denied job opportunities, etc.
"Why do detransitioners exist?"
Detransitioners are typically people who lied, slipped through the cracks, were so sure of themselves they kept pushing until they got what they wanted, or even people who had a separate condition that led them to believe they were trans. (certain personality disorders can cause this, as an example). They're not proof that transitioning is bad and doesn't work anymore than someone who didn't improve with therapy is proof that therapy doesn't work.
"I've been told that he/his aren't always male pronouns and vice versa with she/hers?"
They are. Words have meanings. You can't just say they're not male pronouns when they are because it makes you feel better. Otherwise what's even the point of pronouns in the first place? They exist to assign gender. That's their entire purpose. If you do ever need a gender neutral alternative for whatever reason, we have they. We use the term they as a gender neutral pronoun all the time when we don't know someone's gender, you've probably done it without even noticing. There's no need to shove feelings into grammar.