What is the crucial difference between using 'noindex' and using robots.txt to block a page from search engines?

The crucial difference between using 'noindex' and using robots.txt to block a page from search engines lies in how the search engine bots interact with the page and whether the page is crawled and indexed at all. Using robots.txt to block a page prevents search engine crawlers, like Yandexbot, from even accessing the page. The bot respects the robots.txt directive and will not crawl the page or index its content. However, other websites can still link to that blocked page, and Yandex might still index the URL based on these external links, although without knowing the content of the page. On the other hand, the 'noindex' meta tag or HTTP header is implemented *onthe page itself. This means that Yandexbot will crawl the page, see the 'noindex' directive, and then *notindex the content. The 'noindex' tag tells the search engine: 'crawl this page, but do not include it in the search index'. Therefore, 'noindex' allows the bot to crawl and discover other links on the page, passing link equity to those links, while robots.txt prevents crawling altogether. In short: robots.txt prevents crawling, 'noindex' prevents indexing after crawling.