Three separate things.

The one thing that perplexes a lot of people about the issue of assigned gender and sex is mixing these three things and treating then as one, especially sexuality and gender. These are totally different things, with sexuality being the most different and completely unrelated.

Sexuality is who you are romantically and sexually attracted to. This is who you love, who you want to have sex with, who you get all warm and fuzzy for.

Sex is the biological aspects doctors use to label you as male and female. Sex is determined by the appearance of your outward genitalia, and by that alone used to automatically presume every other biological aspect of your body, such as your hormones and chromosomes.

Gender can be the social construct of masculinity, femininity, other, gender roles as made in the society, gender expression outside of or in terms with social gender constructs, the attribution of your gender expression by society, and lastly but most importantly; the personal and most fundamental identity of gender. Gender is individual, unique, bio-culturally variant and open to analysis. Due to the complexity of this, it will have its own chapter. Isn't it lucky!

So, why assigned gender and sex? What does assign mean?
Assigned is used to express the social application of a certain gender and sex. The assignment of the sexes male or female based on outward genitalia, or forced assignment of either of the sexes in the case of intersex individuals. When a person is assigned a sex of male of female, there is an automatic gender assignment of boy or girl. this later progresses to man or woman. When a person is assigned gender and sex diverse, they don't conform to what was labeled on them as babies and through life.

So, if sex is about biology, a person that is assigned sex diverse is technically female or male right?
Nope, this answer is actually really simple, but it's a long one so grab some tea. I learnt about the variations of different sexes while studying at uni, and it often fascinates me that people will limit the human body to only two sexes. No two bodies are the same, and people often go through life without knowing things like their chromosomes, let alone conforming to only two chromosome combinations. There are as much as thirteen, yes, thirteen, different combinations. Hormones also becoming startlingly varient, and a person with hormone levels that are relevantly "level" for their assigned sex who takes hormones often changes their body into a way that is no longer of their assigned sex.
Sex is often treated like a social construct by scientists, medical professionals and assigned sex diverse persons because it is literally something we, as people, began constructing. We saw that in some cases certain biological attributes matched up, and we applied names to these attribute collections. The problem with this is not just that not everyone fits all attributes, but we most often don't even check and force an assignment. This can be harmful for certain individuals, especially intersex persons, and the sooner we recognise the individuality of our bodies, the quicker we can learn to accept people of all differences.

Also, imagine, intersex people are almost as common as green eyes! That's a lot of people.

Next up, gender!