You haven't read anything about this. It's very clear. The first thing you learn is that sex and gender are different. Sex is biology. Gender is identity.

The second thing you learn is that sex is not binary. (And gender, being a social construct, certainly is not set in stone.) Genes may be XX, but maybe some other factor may be preventing that gene from expressing fully or even at all. This can lead to highly androgynous folks or folks with odd genital configurations. It takes genes, gene expression, and hormones for a human to express characteristics of some sex. Not all three of these are perfectly aligned. You can argue that genes control all of it, but that doesn't stand. Genes can conflict, and environmental factors can affect things.

I learned all that and more in just twenty minutes of reading. Please, go do some homework. Start with "what is the difference between sex and gender," then let the rabbit hole take you down. At least, that's the path that helped me learn a bunch of this stuff.

And regarding Dunning-Kruger, the key point is confidence. That said, I'll caveat all the above I've said with this is just stuff that I've read from sources that I trust, which I can corroborate with my existing knowledge of genetics and broader biology. I'm not an expert. I can be proven wrong. Most of this is definitions and quite simple stuff, so my confidence is high but still shakeable.

Normally, I'm a stickler about answering asked questions, but your questions seem to be based on a misunderstanding of definitions. Once you get that sorted out, we can try again and maybe learn something together