When and how did the current material culture of sleeping in Germany take shape? (Couples share a bed but not blankets, broad flat pillows meant to be folded, etc)

Something I've always found strange as an immigrant in Germany is the customary sleeping arrangement. While couples share a bed, but sleep under separate blankets (the duvets are half the width of the bed and lay side by side). The standard pillows are, I believe, 80x80 cm and to me they resemble sort of quarter-sized slightly thicker duvets more than pillows: they are very loosely packed and lay flat, and the procedure seems to be that you kind of fold/squish/mash them up into a usable shape (I hate them, if you couldn't tell). Then you have more common (but still not universal) features like the use of both sheets and duvets, as well as more traditional accessories like the incorporating the wood or shavings from Pinus cembra either into the bed or little eyepillow like things for its soothing smell, or the little pillows full of cherry pits. I'm interested in all of it but if I had to prioritize... tell me about those stupid formless pillows that are big in all the wrong dimensions and basically nonexistent in the one dimension that matters.