What robots.txt actually controls (and what it doesn't)
robots.txt is a plain text file at the root of your domain that tells crawlers which paths they may request. It's a crawling instruction, not a security control and not an indexing control. Anyone can read yours by visiting /robots.txt, so never list secret paths there — you'd be publishing a map of them.
The critical misunderstanding: blocking a URL in robots.txt does not remove it from Google. If other sites link to a blocked page, Google can still index the URL with no description ("No information is available for this page"). To keep something out of the index, allow crawling and add a noindex meta tag, or require authentication.