Write the paths you want to keep crawlers out of, then test whether a path is actually blocked
The rule with the longer matching path wins. When both are the same length, Allow wins. This tool follows the same rule, so with Disallow: /admin/ and Allow: /admin/help/ the path /admin/help/index.html is allowed.
No. robots.txt only stops crawling. If other sites link to the URL it can still appear in results as a bare URL with no description. To keep a page out of results use a noindex meta tag and do not disallow that page, because a blocked crawler can never read the noindex.
Yes. * matches any sequence and $ anchors the match to the end of the URL. For example Disallow: /*?sessionid= matches URLs with that query, and Disallow: /*.pdf$ matches URLs ending in .pdf. This tool supports both when it tests a path.
Google ignores Crawl-delay. To reduce Google crawl rate, use Search Console instead. Some other crawlers such as Bing do read it, so add it only when you are targeting those.
At the root of the site, for example https://example.com/robots.txt. It is only read at the domain root, so a copy in a subdirectory has no effect, and each subdomain needs its own file.
No. Both the generation and the path test happen entirely in your browser. The paths and URLs you type are never sent to or stored on a server.