How are the margins and the gutter decided?
You don't set margins by hand, and that's deliberate — they're consequences of your book, not preferences.
The gutter grows with the page count
The gutter is the inside margin, next to the spine. A thick book has to be forced open further before the inner edge of the text is readable, so the thicker the book, the more paper the gutter needs.
The formatter steps the gutter up as your page count crosses 150, 300, 500, and 700 pages, and adds a little more on top for wider trims and for styles that want a shorter measure.
Crossing a boundary re-breaks the book
This is the one genuinely book-wide consequence in the whole tool, so it's worth understanding.
The gutter changes the measure — how wide a line of text is. Change the measure and every line in the book breaks somewhere new, which changes the page count, which can change the gutter again. So an edit that nudges a 298-page book to 302 pages doesn't just add two pages; it re-sets the entire book.
The formatter tells you when this happens rather than letting the text move quietly. If your book is sitting within a few pages of 150, 300, 500, or 700, expect the front matter you add later to have a bigger effect than its own page count. Add it before you settle on a final number.
The outside margin grows with the trim
A 6 × 9 page is a full inch wider than a 5 × 8 page, and an inch of paper is about sixteen characters. A single fixed outside margin can't hold a sensible line length across that range, so the outside margin opens up as the page gets wider — the adjustment a designer would make by hand.
The result is that every style lands inside the trade norm of roughly 60 to 72 characters a line at every trim. Your export report prints the number your book actually got.
One file, both printers
There is no printer setting on the interior, on purpose. Two nearly identical PDFs is a choice an author can get wrong at upload, and the cost of getting it wrong is a rejection.
Instead the interior is composed to the stricter of the two printers we build for at every page count: KDP publishes a gutter table that starts at 0.375 in, IngramSpark asks for at least 0.5 in on all sides of the text block, so the floor is 0.5 in. The same file uploads to both.
If you're printing with Lulu or B&N Press, note that their formatting guides ask for a wider inside margin than either KDP or IngramSpark at some page counts. The Cover Maker builds wraps for them, but the interior is not composed against their tables — check their current guide against the margins printed in your export report before you upload.