It’s not.
It’s determined at conception..............
Correct. Your genetic makeup is set when a particular sperm fuses with the ovum at conception. Birth happens much later.
Absolutely correct! Male genitalia will not develop unless the fetus is XY. In addition, no matter what the child/adult “thinks” they are and no matter what hormones they take or surgeries they undergo, the nucleus of every cell in their body (except the eggs or sperm which can have only one sex chromosome) will contain an XY or an XX. There is NO way to change what is inside the nucleus of every cell in the body! IMO, parents who encourage their children to be “trans” are committing child abuse!