It's inside vagina
The "graffenburg spot" or "g" spot is in the upper portion of the vagina just in front of the cervix
and under the bladder
It forms during 2nd stage female sexual excitement as a firm "tented" spot. It can be felt during that stage by placing a finger inside the vagina and making a "come hither movement" with that finger. It provides pleasure in additon to the clitoris.