Where are the front cover, back cover, and spine placed in a book layout in InDesign?

To add to Peter's excellent advice, you need to consider whether the cover is for a paperback or a hardback book. If the latter, you'll need to allow for flaps.

For those producing book covers regularly, there's a great plug-in which saves a lot of time: https://www.danrodney.com/scripts/makebookjacket.html

Other than for fairly simply booklets and saddle-stitched catalogs and the like, a book cover should be a separate file, usually composed as a single wrap-around sheet. Covers are usually printed separately from the book body, often on a completely different press/line.

There are two basic ways to lay out a cover file and many details involved for each. As noted, the best resource you can use is a template from your printer, especially if it's generated for your book's specific size (trim size, spine thickness based on page count, etc). You can often lay a template image right into an InDesign layer and use it as a visual guide to your design and placement.

I should add that you really need to consult with the printer on this. Most likely they will provide either a template, or a written guide, specifying the correct spine width for your book. This dimesnsion must be calculated based on the paper stock and number of sheets required for the book block (and there could be blank pages you haven't figured on due to press sheet and signature size combinations)

If the cover is to be printed on a differnt stock from the corpus, then it maks sense to use a separate file.