Nullification removes external genitalia while preserving a urethral opening and anal opening.
The urethral opening is flush with the skin. So if there was a penis, the urethra is shortened to the base of the pre-existing penis. If there was originally a vagina, the skin edges meet the original urethral opening.
Original with Penis: penis, both testes and scrotum removed.
Original with Vagina: vagina, clitoris and labia removed.