What leads to the different types of navels?
After childbirth, the umbilical cord is either tied (similar to a tourniquet - the cord itself is not tied into a knot) or clamped to cut off the blood supply. It is then cut distal to the clamp/tie, separating the child from the rest of the cord and the attached placenta. The remaining cells die and dessicate, and the stump eventually falls off, forming the umbilicus or belly button. The default result, barring any issues, is an "innie". However, an "outie" may form if the child was born with a tiny umbilical hernia, or there was a small unnoticed infection at the base of the umbilical cord. Either of these could lead to the formation granulation tissue, described colloquially as an "outie". People can also have combination navels, with the indentation of an "innie" but a small amount of protruding granulation tissue inside.