1. sex and gender are different things.

2. Trans women were biological males. But sex is determined by a lot of factors, including hormones. Trans folks often use man made chemicals to trigger sexual development into the sex they wanted.

Think of sex like a slider bar on a screen, with "male" on one side and "female" on the other. You start somewhere on the scale. With hormones you can put your finger on the slider and schwoop that lil sucker anywhere you want. It's not just for trans folks though! Straights use the slider too. Wanna be more butch than you already are as a guy? Add testosterone! Going through menopause? Add estrogen!

The impact of opposite sex hormones to a trans person's already developed sex organs is mostly to render them useless, but that's fine, straights do that too, mostly on purpose. That's what adoption is for. If a trans person wants to go all in on the change, they can get the reverse-that-thang surgery and turn that outie into an innie.

At that point, there's like... Zero difference between a "natural born" woman who had a hysterectomy for some reason, and a trans woman.

It's all very science fiction when you get into the details.