Crawling fetches the page; indexing decides whether to keep it and under which content. Google indexes a page when it is reachable, returns a 200 status, is not blocked by a noindex rule, is not a duplicate of another page, and carries enough original content to be worth keeping. Thin or duplicated pages are often crawled and then left out.
Search Console's page indexing report shows, for a site, which pages are indexed and why the others are not. AI assistants that retrieve sources from a search index inherit the same limit: a page outside the index is outside their answers.