prefacing this with the fact this is just my personal opinion, i am aware not everyone will agree with my viewpoint, but its one garnered simply from being part of a large social scene that is LGBTQIA+ heavy, with the whole rainbow of sexualities and gender identities and a lot of conversations with people.
our Sex is biological.
Our Gender is mental/personal/ephemeral.
When we are born we are assumed to be the sex our genitals present as most closely. As that sex, it is assumed, as a child (For most parents) that their child will identify as the same Gender as our apparent biological sex.
However, as we mature and gain sense of 'self' for some people that simply doesn't flow. For some people, regardless of reason, when they think of themselves, their sense of 'self' doesn't match the societal accepted binary of their sex's matching Gender identity.
For some people, its to the point that they feel a strong disconnect between their sense of their Self and the genitals they possess, which is where Trans-gender comes into it.
There is a vast spectrum between Cis and Trans (using the words just as a descriptor) and most people will sit somewhere on that line, and todays society allows for that gender fluidity to be matched in how we present ourselves outwardly in our daily lives.
The vast majority of people who do not identify as the same gender as their outward biological sex, just want to get on with their lives and be accepted as the gender that matches their inward sense of self.