@Charlie11223
On sex and gender, from a biologist point of view:
Sex is biological and determined by your chromosomes- in mammals this is XX for females and XY for males. This will affect your reproductive biology- so XX placental mammals have a womb and lactate, XY mammals have testes and a penis and produce sperm. Some sexes may look obviously different due to secondary sexual characteristics- often used by males for fighting or attracting mates, but in some species the sexes may look very similar from a distance.
In some animals (such as reptiles) sex is determined by factors other than chromosomes. All animals that reproduce sexually have a biological sex- the female produces the eggs (or the larger gamete/sex cell) and the male produces the sperm (or the smaller gamete/sex cell).
In some species, such as clownfish, individuals naturally change sex including all their reproductive biology- this is known as sequential haemaphroditism. It does not occur in mammals.
Some humans (and animals of other species) are born with intersex conditions, meaning they have abnormal sex chromosomes or their secondary sexual characteristics do not match their chromosomes. There are lots of different intersex conditions which can occur in humans, but most are extremely rare.
From a biological perspective it is arguably impossible for a human to change sex, as they cannot change their chromosomes or produce the gametes of the other sex.
Gender is a term that only really applies to humans and refers to behavioural/societal secondary sexual characteristics. Many species have large behavioural differences between the sexes. In humans gender can refer to behavioural differences, lifestyle differences, differences in dress and appearance etc.
In humans, it is thought that some of these differences are caused by biology- e.g. testosterone makes males more aggressive, and some are caused by society e.g. in modern western society it would be unusual to see a male wearing a dress and high heels.
Gender differences are very hard to define, as they change over time- for example in the regency period it was very common to see men wearing make-up, and there are large geographical differences as well (although globalization does have an effect on this). However, in the western world, you can still stereotype the two genders to certain extent. Behavioural traits are part of gender- e.g. men are more likely to be assertive, women are more likely to appease- although this arguably changing in modern society.
Many people view gender as entirely a social construct. As such, you could obviously create more genders within society- but historically in the western world, there have only really been two genders.
Obviously, it is possible to superficially change gender- however it is arguably harder for transgender people to change their behavioural traits to match that of their chosen gender.
I think it's really important to use these terms for their correct meaning, and in biology it is considered very important to use sex when you mean biological sex.