Gender- a psychological and social spectrum based on the correlation between how someone expresses themself and the societal norms of where they are in the world
Sex- the biological and genetic characteristics of someone. This includes the XY/XX chromosomes, estrogen/testosterone production in their body, and their genitalia.
There are 3 sexes: male, female, and intersex. An individual who's intersex has both male and female biological/genetic characteristics. The amount of gender identities, however, varies in different parts of the world.
Sometimes, one's gender and sex aren't aligned, so they make changes to their body so that it fits them better. Aka become transgender
This is what they should have in the textbooks.