A search index is the vast database a search engine actually draws from when it returns results, the collection of every page it has crawled, evaluated, and decided is worth storing and potentially surfacing for a query. Ranking is a separate step that happens after: a page has to already be in the index before it's even eligible to compete for a position on any results page.
Getting into the index isn't automatic just from being crawled. A search engine can crawl a page and still leave it out if the content is judged too thin, duplicative, or low-value relative to what's already indexed, or if a noindex tag explicitly says to keep it out. robots.txt affects whether a page gets crawled in the first place, which indirectly affects indexing, but noindex is the direct, reliable tool for controlling what actually makes it into the index versus what doesn't. Google Search Console's URL Inspection tool reports a page's current indexing status and, when it's excluded, the specific reason why, which makes it the fastest way to diagnose a page that isn't showing up in search the way it should.
