My booklet’s back cover should not be numbered. How should I prevent the number? Do I use sectioning? Do I use Format | Page? Do I use ‘Page Styles’? (If so, how do I use those exactly?) Is there some built-in “back cover / last page” style?
Use a page style. The Default page style has no header/footer and so can be used as the basis of a custom page style (e.g., Last) for this purpose.