The belly button is where your umbilical cord was connected to you when you were a baby and in your mothers womb.
The umbilical cord where your belly button is carried nutrients and fed you while you were in your mothers womb so yes the belly button is connected still inside your body even as an adult.
The belly button is connected to your liver and the bladder via ligaments and veins.
Your bladder and live is still connected to the umbilical cord even as an adult although nothing flows out of your belly button because it's closed up after birth.
Here's a diagram of how the umbilical cord connects inside the babies and adults body.
