It doesn´t go anywhere. First off, some men try to fool themselves about penis length. The lower part of a penis shaft, near the legs isn´t really part of the length you should count when going in a vagina. Even if you´re thrusting as deep as you can, the base of your penis might give the illusion of getting in, but it´s not. The base is thicker and encounters resistance on the labia (lips of the vulva). Also, the head of the penis squashes down when it gets inside anyway, if it manages to hit the uterine cervix, which is the limit of how far it can go. Also, consider that the vagina is not precisely straight and it´s not a pipe shape. It´s more a cavern. The erect penis actually bends inside to some degree. Yep…the penis literally squashes up in there a little and that reduces the effective length.
When you think you´re penis is all the way in the vagina, in a straight line, it´s really not.