Which comforter should I buy?

When I stay with my friends or grandparents they always have those super cozy down comforters and a blanket that goes on top. What is the difference between a down comforter and a duvet cover? Also theres like a million different choices. I know high thread count is softer but theres ...Egyptian thread count , Siberian Down comforters, Goose down comforters, Theres some "oversized" and some are "High loft". Im overwhelmed whats the difference?