BTW▸ , I am not very keen on duvets either, dodgy foreign invention. Did not some notable person state that duvets, ice cold "beer" and the like, "should be sent back to the continent where they belong"
No nation that uses duvets has ever won a major war.
The proper bedding for an Englishman is wool blankets.
Does the speaker in the house of lords sit upon a folded duvet ? no of course not! he sits upon the WOOLsack.
Blankets will only get you so far - as I remember from the 50s, lying in bed on nights when there was ice in the inside of the windows, you needed to add a quilt - or eiderdown - or even a counterpane. All of which words, like duvet, show how closely linked bedding vocabulary (and maybe usage) is in French and English.
Duvet in French means down (whether eider or other bird). It's actually cognate with down, from Old Norse
dunn.
The object in French is a
couette - meaning quilt. Again, these are the same word, from the Latin
culcita. (And, if you go back far enough, feather quilts were used under as well as over the sleeper, to soften a straw mattress.)
Another old term for a quilted bedcover is
courte pointe, literally short stitch. While it's tempting to see that as the origin (you pull a stitch tight through the quilt) it actually comes from the same source as
couette.
Courtepointe meaning a bed quilt was copied by Mrs. Malaprop's Bedding Company as counterpane.
We use eiderdown, in a way that parallels
duvet, to mean a bed quilt. In repayment for counterpane we gave the French that one as
édredon.
I suspect the history of bed quilts may involve their use without blankets more often that you imagine. After all, the key innovation was just to add a washable loose cover, but sheets can be used instead. Of course the first duvets we had were called continental quilts, and seen as more Scandinavian than French - I suspect they only took them up in a big way around the time we did. So why the change in name, to a word that doesn't mean that anyway?
But as to why French and German pillows are such a silly shape ...