It varies by product: a duvet cover alone, a cover with pillowcases, or a cover with pillowcases and a fitted sheet. Read the contents list on the product page rather than assuming.

Almost always mattress depth. Thickness varies a lot and a sheet cut for a slim mattress will not stay on a deep one, especially with a topper added.

From the top surface down to where the base begins, including any topper you use. Compare that figure to the depth stated on the product page.

The duvet size. Covers are sized to the duvet inside, and many people use a larger duvet than their bed for extra overhang.

Because a made bed is seen from above and one side, and much of the surface disappears down the sides and under pillows. Large open designs survive that; dense small motifs do not.

Inside out, fastening closed, cool wash, low dry. Printed covers fade through heat and friction rather than detergent, so temperature matters more than what you wash with.

The filling will shift inside and the bed looks untidy. Too small is worse, since it compresses the duvet and reduces warmth. Matching the duvet size avoids both.