The tool fetches the sitemap as a plain XML file — it does not render or crawl any pages — and parses the <loc> and <lastmod> of every <url> entry. If the file turns out to be a <sitemapindex>, it fetches up to five of the child sitemaps it points at, in parallel, and merges their entries into one list. Gzipped files are decompressed automatically, and plain-text sitemaps and RSS/Atom feeds are read as well, since crawlers accept those formats too.
Reading the results
The headline count is the number of URLs in the list; if the sitemap holds more than the 2,000-URL cap, the summary shows the real total so you know how much is missing. Child sitemaps that could not be read — a 404, a timeout, or a nested index — are listed with the reason. Copy all puts one URL per line on the clipboard, ready for a bulk checker; Download CSV gives you a url,lastmod file for a spreadsheet. To check the file itself for structural problems rather than list its contents, use the Sitemap Validator.