I have made and used snow caves before (much easier to make than an ingloo - especially for just a day or three). They are amazing for quick shelter from the cold.
The snow/ice inside does stay a little below freezing, but the room does get up to a pretty comfortable temp - just with body heat. I've had temps up into the low 50's (F) inside with two people before - and -20 (F) outside! The inside does start to melt a bit and glaze over. But the outside temps keep it from melting more than just a bit on the inside. A key point to remember, tho, is to keep a fresh air hole open. CO2 buildup and oxygen depletion can be a concern.
Yes, you do need an "insulating layer" on top of the ice/snow inside where you want to sit/sleep - just like most any time outside. But you only need to insulate from around 25 to 30 degrees (F), not that -20 (F) temp outside.
I have slept out in below freezing temps with only two wool blankets before. Yes, I built up an insulating layer from the ground with leaves/grass. And I have done so many times - and usually without a fire! I don't like to NEED a fire to survive the night. It's a nice comfort to have, but having to rely upon it adds a whole extra level of stress to the experience. I just plan carefully, block any wind/breeze/draft, insulate from the ground, wear my regular clothing to bed, change to dry socks (or clothes if necessary), and mentally prepare myself for it. Good planning and prep got me through the nights more or less comfortably. Obviously, it didn't ... kill me off!
But then, I'm just that grumpy ol' neanderthal caveman living out in the hills. So my experiences will be different from those living in the cities in those climate-controlled "hives".
Other's experiences may vary ...
Mikey - yee ol' grumpy blacksmith out in the Hinterlands
p.s. The concept of a sleeping "bag" seems to be something of a more modern idea. The historical research I have done in the 1600's through 1800's has shown that most people just used "blankets" - made of animal skins or wool. They just tended to lay and fold them together as needed - but didn't actually "sew" them up into a bag. Ditto the research I have done on Indian cultures. Altho, I do tend to fold my two wool blankets into something that might be viewed as a "bag". I lay one out flat. Then I fold the other in half lengthwise, and lay it on the first along one edge with the folded edges towards the middle of the first blanket. I then fold the other half of the first blanket over the second. At this point I fold under the bottom edges and pin them together. And then I wrap it all up in a tarp - to create my final bedroll. So this does create something of a sleeping "bag", but I can vary it as needed each time I head out.
p.p.s. Yes, my bedroll and camp gear is heavy. But it is also documented to what people used/carried back in the 1700's. So that's what I carry and use, and just live with the weight. That's one of the reasons I and my friends like winter camping/trekking. We then have and use a toboggan (traine) to haul our gear in. And why we also like canoe or batteau trips. But we still have to carefully watch how much extra ... stuff ... we drag along!