Reddit mentions of Luxurious King/California King Size Siberian Goose Down Comforter, 1200 Thread Count 100% Egyptian Cotton 750FP, 50oz, 1200TC, White Solid

I see someone explained sheets below.

In America, duvet (sounds like "doovay") and comforter are often used interchangeably, but a traditional duvet is stuffed with feathers. They are fluffy and puffy, like a stuffed winter coat, instead of flat like a quilt.

Here's an example:

https://www.amazon.com/Luxurious-California-Siberian-Comforter-Egyptian/dp/B01GUZ08MI

Flufffff. This one is expensive because it's made with down feathers and thicc. But you can also get them in lighter weights and with synthetic fillings (synthetics can be cheaper, easier on allergies, and easier to wash, too.)

Ikea calls them all comforters now and has a lot of affordable ones:

https://www.ikea.com/us/en/search/products/?q=comforter%20

But whatever you get, they are plain because they are meant to get a duvet cover. The cover is just like a big pillowcase. It goes on the duvet/comforter so you don't get it gross so fast. You wash the cover every week or two when you wash sheets and pillowcases. You wash the duvet (and your whole actual pillow) like once every 3-6 months.

If you have a bed for two, you buy:

A sheet set

• one fitted sheet
  
• one flat sheet
  
• two pillowcases
  
  And
  
  A duvet cover set
  
• one duvet cover
  
• two pillow covers, sometimes called shams
  
  So you end with with two pillows to match the sheets, two to match the blanket. Like a fancy person!
  
  
  If you have a simple quilt or one of those comforters like you get at Walmart that's just colored cotton with stitching, then you just use the flat sheet. So you do the fitted sheet, then put the flat sheet on top (tuck it at the bottom). Then your blanket. It'll look nice when you turn the bed down and keep your body oil from grossing up your blanket so quickly.
  
  Here's an easy video with basics:
  
  https://youtu.be/tuIZ6c9bDfg