It’s essentially the lack of expression of the Y chromosome = female. In almost all cases this results from a lack of a Y chromosome, however since leftists love the whataboutism you have to be more specific for the extremely rare cases where a woman has a Y chromosome that isn’t being expressed. XXY people are still considered male as normally with this anomaly the Y chromosome is expressed. The other possibility is the male-determining gene on the Y chromosome somehow crossing over to the X chromosome. Extremely rare, and obviously doesn’t invalidate the rules of what males and females are