One time I stored a bag of maple leaves in a garbage bag which I used for feeding my compost. I didn't need it much over winter, and in spring when I went to use it, dozens of bumblebees came out. They'd hibernated in a bag of leaves. It was such a cold winter for our climate (it hit -15°C one night!) and somehow they were just fine.

When I was a kid I didn't think much about where they hibernate, how, or why. But they're definitely a species that continually yields fascinating revelations. Apart from their ability to sleep in leaves for 6 months or so, they're also able to learn to use door flaps and, apparently, survive flooding. They're resilient little creatures.

Every animal seems to have surprising abilities and behaviours if you're just lucky enough to see it.

https://www.youtube.com/watch?v=1J9Cr_M5osI

That's how endless insects, ones genetically design to survive our winters, do so. They crawl under leaves and dying grass, which insulates them from the cold a bit. Their bodies can freeze and thaw, and they'll be fine.

If you watch robins in the spring, after the snow melts but before the ground thaws, you'll see them turning over leaves to find and eat the insects. I see a lot of this, because I have a lot of trees (rural property, with forest around me). Often there are robins migrating, who stop and fill up thanks to my lawn and its plentiful ground leaf cover.

Decomposing and decaying organic material often generates heat (compost piles sometimes spontaneously catch on fire due to this). The bees may have survived due to that or maybe they were attracted to that in the first place.