There are a lot of articles around on this.
One article says it’s for comfort;
This resting position calms and centers the mind, making this therapeutic position conducive to stress relief. By laying in this position, it relieves the tension a little one’s body has absorbed throughout the day.

It turns out there are several reasons science has posited to explain why babies sleep in child’s pose. The reason babies who sleep on their stomachs have their butt up in the air is due to their position in the uterus. … Many babies will also tuck their arms between their chest and the mattress as they were positioned in the womb. It’s comforting to them.
It looks very cute indeed !

I have no idea why, both mine did it, I assume it’s comfortable. They look pretty snuggley when they sleep in that position

My first daughter did it all the time, used to crawl out of the blankets and pajama bottoms and then sleep on top of the lot with her little butt in the air.
So adorable.

My baby will quite happily sleep sweatily on her tummy too.
Perhaps they put their bottoms in the air to mimic the position they were in whilst in the womb.

Good question! My daughter tends to do it when she is sick with a cold/flu but not really at other times. I read a book that said babies will sleep on their stomachs if they are cold but I found this to be untrue for my daughter (she’d be sweating and sleeping on her belly).