There are different ways of setting up a bed with a duvet. In the United States a bed would most commonly have a fitted sheet on the mattress, then a flat sheet on top of the fitted sheet, tucked under the mattress at the foot of the bed. You sleep on top of the fitted sheet and under the flat sheet. The duvet goes on
top of the flat sheet, so it basically doesn’t touch anybody in the bed.
The other way is to eliminate the flat sheet, and have the duvet inside a regularly-changed cover, because, without the flat sheet, the duvet is in direct contact with the people in the bed.
A more detailed discussion can be found at: https://www.realsimple.com/home-organizing/bedding-debate-to...