It’s hooked to the placenta. Nourishment from the placenta flows down the umbillical cord into the baby.
When the baby is born, and the cord is cut it begins to dry out. A scar forms under it. That scar is the belly button.
After a couple of weeks it drops off.
I remember impatiently waiting for my babie’s cord to drop off so I could take them swimming!
The mother also has to “deliver” the placenta after the baby is born.