So yes, chromosomes determine your sex. Unless you're intersex, which is a disorder and nothing to do with trans. (Don't get sucked into that argument, it's pointless. The intersex society has publicly said for TRAs to not appropriate them to shore up their argument).
So chromosomes determine sex.
Why does it matter?
Several reasons. The most pressing of which is that transactivists are pushing for a law change.
Which means that anyone can self identify as a man or woman, quite legally, by filling out a form. No diagnosis of gender dysphoria, no change in appearance, nothing.
The extent to which this can be exploited seems to occur to just about every woman in the country, but not the government or the opposition.
Despite the fact that labours woman officer post is currently held by 19-year-old boy, who thinks biology is irrelevant and he can get pregnant.
Despite the fact that they have forced women's aid to consider whether or not a man who thinks he is a woman should become rape crisis counsellor.
Despite the fact that all women shortlists could be made up entirely of men, identifying as women.
And despite the fact that the legal fiction that a man is a woman will give him an automatic transfer to a woman's prison, despite being a double rapist. Which has just happened
www.google.co.uk/amp/s/www.thesun.co.uk/news/4400327/transgender-rapist-womens-prison/amp/