While a dog's belly button looks notably different from a human's, it gets there the same way. Like babies, puppies in the womb get the oxygen and nutrients they need to survive and grow by an umbilical cord, which connects to the abdomen. Because the pup no longer needs that once he is born, it falls off, leaving behind a mark on his belly that he'll have his whole life.
Umbilical Cord Scar
The dog's belly button isn't a pronounced cavity like a human's. Rather, it's a small scar that is flush with the skin. When he is born and the umbilical cord falls off, he is left with a scab that heals into a scar, which you can see on his belly's skin. If your dog is laying on his back, look at his naked belly skin in the middle of the fur line closest to his head -- the widow's peak area, right below his ribcage. This is where you'll see the white, hairless scar where the umbilical cord was once connected, sustaining him while he was in utero. It won't be white for long.