Seeing your cat passed out and belly up should make you smile because it’s almost unbearably cute and clear that your cat is more than comfortable in your home.
In some cases, cats are looking for a specific type of attention in the form of belly rubs.
It’s possible that over time some cats just develop a preference for sleeping on their back as opposed to other positions.
I don’t think the specific position of laying on the back has that much significance for kittens.
Yes, you’re right about that, but that’s only applicable when cats are actually awake!