The dog's sense of smell is very sensitive. A newborn baby will have the smell of its owner. The dog knows that this is the owner's child. In addition, the owner's usual attitude towards children will always tell the dog that this is your little master. It will make the dog realize the status of its little owner; therefore, it will not bite, but choose to protect family members and take care of the baby.
And the dog is very smart. The baby is about the same size as it and does not emit any threatening smell. The dog will not behave aggressively.