Here's a thing that really helped me understand it.
I was asked to think about what it takes to be a man. Is it chest hair? P**is & testicles? Having a beard? Excessive nose and back hair?
Now, we're going to "pluck" your brain out of your body. And then put it in a robot body that isn't even human-looking. A straight-up star wars droid that doesn't even have arms or legs. Just rolls around on whatever.
If I were asked to identify myself when filling out a job application while in the robot body, would I still check off "male" on the employment form?
My answer was yes. I'm a dude, even in a non-human robot body.
And that's the point- for trans people, that inside self does not match their physical exterior. If you were to pluck their brains out of their bodies and put them into that same robot, they would pick the internal sense of self on the employment form.
so, think about what it is about you that says what gender you are. Then imagine if your brain was plucked from a human body, would you still be that same gender? Or would it be different?