I will give you the cliff notes version. There’s a thing called an umbilical cord. It’s used to provide nutrients and has been used as a mode of nutrient supply in placental mammals throughout evolutionary time.

The problem is that people who start with the conclusion (“god made people special”) get all fouled up when they have to walk through increasingly convoluted thinking to make their presuppositions line up with reality. Belly buttons are only a problem is you start from the idea of some weird mythology. Otherwise, they’re just expected and fully explained. It’s really not a matter for a course, but if you’re interested in a freshman level text on biology I could probably recommend one.