IME lots of people just try stuff the duvet in and then shake it to get it in the right place.
</ducks>
I think my parents' generation viewed duvets as unacceptably decadent, and thought all children should learn to fold hospital corners[0] to build character.
Also, many rooms let you get the temperature to ~15C.
Here is a video of someone actually folding the sheet as the parent requested: https://m.youtube.com/watch?v=ckTCocBCUN4
https://www.amazon.com/Home-Comforts-Science-Keeping-House/d...