A paperback cover is one PDF: back, spine and front, plus 0.125 in bleed all round. The spine width depends on page count and paper type, so finish the interior first, then use KDP's cover calculator or template for the exact size.
Spine text
KDP only allows spine text when the book is thick enough (around 80 pages or more). Keep it centred with a little margin.
Ebook cover
The Kindle cover is just the front, as a JPEG or TIFF. A 1.6:1 ratio such as 2560 × 1600 px is the usual recommendation.
In Author Magic
The cover studio builds the full wrap from your final page count, so the spine matches the interior.
KDP's rules change from time to time. Check KDP's own help pages for the current requirements before you publish.