There is a lot of confusion about sex and gender, especially if people use the term "gender" to refer to things that are separate.
Gender / sex has three aspects :

1. the biological sex of a person. There are exactly two of them (male and female), you are born with it, you can't change it (no mammal can do this) and it is as objective and real as the five fingers of a hand or your blood group- (or can anybody change their blood group via self-ID?).
2. the gender role, also called 'gender': it is socially constructed, it is fluid, there is a spectrum (masculine vs feminine) and there are no limits to the imagination.
3. the legal sex. This usually coincides with the biological one, except in those rare cases where someone suffers from gender dysphoria, and thereupon applies to be allowed to make a transition. Or if someone is intersex, in which case they can ask to become non-binary (or 'diverse', as it is called in Germany)