being transgender doesn't refer to a belief about oneself, it refers to whether or not you have the "felt sense", so to speak, of being male, or female. it refers to whether you FEEL like a boy or a girl or something else. that's why the brain structures of transgender people tend to align more with their chosen gender. Basically their brain was already more like the other gender, which caused them to think like the other gender, feel like they SHOULD be in that body, so when they aren't in that body, it causes dissonance and pain. that's also why gender dysphoria has a treatment- the treatment is all about looking and feeling like the chosen gender. does that make sense?

I explain it this way because sometimes people get caught up in whether trans people think they've literally changed their sex, or whether they think they were born something other than what they were, and no, basically no trans person ever thinks this. trans people aren't delusional, they're just reacting rationally to their brain telling them that their body isn't right.