[wanorris]

As people who have been in certain kinds of accidents or been treated for certain kinds of medical conditions can tell you, having a certain set of body parts is not a requirement for being a member of either gender.

The overwhelming majority of people are born with a certain set of chromosomes, a set of parts between their legs, and an idea in their head that all agree with one another on what gender they are. It's a large enough majority that it's easy to think that these are the same thing, because for most people, they are.

But for trans people, these things disagree, and the gender that they think of themselves as in their heads wins. Since they are being treated by the world at large as something they don't consider themselves to be, they take steps to correct how the world views them by transitioning. Given money, access to treatment, and other constraints, this may include surgery, hormones, etc. But even in the absence of these things, they can still be trans people who know what their true gender is.