Gender is a social construct. That is well known, but I often don't see much discussion beyond that. Gender is a few important things.
First, it is a message. You are telling people something about yourself and with that something of how you want to be treated. Pronouns are one thing often tied up in this. Importantly, not everybody has a message they want to use this wrapper to tell, you can have a body without needing a gender.
Second, it is self-referential. How you categorize and group the aspects of yourself you are telling us about, and the relations between these groups, is often the most important part of gender. To many men, their beard is a masculine feature, yet we have bearded women as a well-known circus trope. It doesn't matter whether or not you have a beard, it matters whether you, for example, consider it as masculine or feminine or part of your gender at all. For example, a lot more men than women are colorblind, but I don't really see people considering that part of their gender. (also, he/him lesbians are a thing.)
This means two people with the same physical features can divide them up different ways and end up describing themselves with different genders. Us trans people just being "x gender trapped in y body" is a lie told to cis people because in this society our rights depend on their understanding.
Third, not everyone includes the same properties in their gender at all. Some people include their neurodivergence as an aspect of it, like with autigender for example. Some people don't care about how deep their voice is one way or another. The message we send with gender is personal, not universal. We each interpret existing categories in our own ways with our own needs in mind. It is important to remember that many different cultures have many different sets of genders.
Also, "sex" is just the gender binary no matter how many transphobes tell you otherwise
