Quick and easy:

Belly buttons are generally seen as completely luck based, and have little to no genetic inheritance. When you're born, your umbilical cord is cut, and part of it remains. The umbilical stump will fall off by itself in a matter of time, and while there are few conditions that can result in predictable results, there is no evolutionary or genetic underpinning to what you end up with.

Both umbilical hernia and granuloma can result in outie belly buttons, and temporary increases in intestinal wall pressure can result in outies. Things like pregnancy can create temporary outies that return to innie belly buttons in time.

To directly answer - belly button formation has been overwhelmingly random as long as umbilical cord removal has existed in pregnancies.

Surprisingly, some people can be born without belly buttons. Conditions that cause growth of the intestines to form outside of the abdominal wall, which results in your umbilical cord not falling off, and no scarring results because of this.

Source: https://education.nationalgeographic.org/resource/belly-button-biodiversity