Hair Follicles

Every human has got the same number of hair follicles on their face and head, give or take, regardless of sex. Your hormones dictate which ones will activate, which is why trans men can grow facial hair. The follicles are there, they’re just not stimulated to grow unless additional testosterone is introduced.

Similarly, a lot of trans women, who were going bald when they were men will start growing a full head of hair when they start reducing testosterone, which revives the dormant follicles and introducing estrogen stimulates the follicles that have never been active. Hair follicles take a very long time to die, and some are never active, to begin with.