Human embryos (and probably others) start out female, and grow the male parts a fee weeks later. There is probably someone lurking who can give better details but that’s the gist of it. Thats why men have nipples.
Your penis is an enlarged clitoris and your testicles are ovaries that fell out.
Yeah the Y chromosome doesn’t kick in until later in fetal development.
There’s actually a surprisingly small amount of biological information stored on the Y chromosome, IIRC the vast majority lies on the X.
You pretty much nailed it.
Also, it gets much more complicated than that; at least 2% of humans are intersex, meaning they’re a mix of male and female, everywhere from outward presentation (rare) to chromosomal (more common).
It’s not just gender that’s a spectrum: sex is, too. I said ‘at least’ because it’s possible for people to go their whole lives without realising they’re intersex. 2% is huge: if you know 100 people, statistically 2 of them are intersex. Most of us know more than that.
The ‘only two genders’ thing is biologically bullshit. Biology is never cut and dried.
eta: for reference, I have a disorder that’s been in the spotlight lately – Ehlers-Danlos – and you’ve probably at least heard of it. You’ve likely talked to someone who has it recently. It occurs in 1 in 20,000 people. Again, roughly 1 in 50 have intersex traits. I wish this had the visibility it deserves. It’s exceedingly normal.