The cover editor checks a paperback or hardcover cover in three ways, before you order a printed proof copy, or where Amazon doesn't send them.
3D proof: the cover on a book of its true size: the trim, and a spine and pages as thick as the page count and paper make them, with a hardcover's boards. Drag to turn it, and scroll or pinch to zoom down to the print's own pixels. Actual size shows it as large as it will be in your hand, once you measure the screen against a bank card. Where the cut may land moves the cover the way KDP's trimming may, up to 1/8 inch either way, so you can see what a spine or an edge could lose. Where KDP adds the barcode, the proof shows its white box.
Loupe: over the flat cover, the loupe shows what is under the pointer at print resolution, 300 pixels an inch, as the PDF is made, with each printed pixel two or four times as large. Look at small type, thin lines and the artwork's detail: what is soft or jagged there will print that way. With the keyboard, select the cover and move the loupe with the arrow keys.
Colours as printed: printing mixes four inks, which can't reach every colour a screen shows. This shows roughly how the cover changes: very bright blues, greens and oranges print duller, and the deepest black prints lighter, more so under matte laminate than glossy. Choose the laminate you'll pick in KDP. It's a guide to which colours will change, not the exact print: only a proof copy shows that.