Because duvets are not customary in America, a flat sheet is put under the blankets to protect the blankets. Taking up the same principal of the duvet cover.
If you have a duvet with a cover, you don't need the top sheet.
Why? Because that kind of set is the standard for US bed linens. Europe used to have greatly varied sizes in mattresses so fitted sheets were impractical. Bed sizes have been standardized here for ay least a century, and most people prefer a fitted bottom sheet. the flat t sheet (which you describe as a "blanket") is used here because we don't use duvets like they do in Europe. The top sheet keeps the blankets and quilts clean and out of contact with the sleeper's bodies.
Are you from Europe, where the duvet cover works the same way a top sheet does? Instead of washing a sheet, you wash the duvet cover. Duvets weren't used much in North America until recently, and having a top sheet meant you didn't have to wash your blankets or comforter very often.
Yes one under and one over. Easier to resheet the bed than wash everything on a regular basis.
