Sex is determined by genetic inheritance. There are very, very few people born intersex due to chromosomal abnormalities.
From that stem primary sex characteristics eg genitals, and hormone influenced development. At puberty secondary characteristics develop.
There is a difference between male and female development, muscle mass, internal systems eg renal/urinary (hence normal range difference), height, voice, hair distribution, etc.
You cannot, scientifically, change your genetic makeup. You can, through surgery and hormone/drug therapy, change the expression of your primary and secondary characteristics to an extent.
You can change the gender you use/identify with. That, as a pp stated is a very complex mix of social norms, culture, learned behaviour, your sense of self and your sex.
I support trans people to the hilt to change their gender, and to live without challenge, discrimination and violence.
I do however have concerns about self ID, and the impact that has on women and girls who have experienced casual sexism every day, who have lived experience of harassment (95% of us) and those who are victims of sexual violence, hold or are subject to religious beliefs which require segregation. This is not bullying, nor transphobia. It is a concern that I see, a wish that a debate which should be about balancing rights is now a fight to suppress rights.
I am a woman. I do not identify as a ciswoman, nor a person who menstruates, nor a vagina-haver. A trans woman may be a woman, but she cannot be a woman in the same biological way as me. She may be subject to the joys of being a woman in experiencing sexism, male entitlement, body shaming and I’m on her side. But biologically her body isnt capable of things mine is, and hers may be capable of things mine is not in strength or sexual acts, and I don’t see why this can’t be acknowledged and discussed.