User-Agent Parser breaks a browser's User-Agent string into readable parts: the browser and its major version, the engine, the operating system, and a device-type guess.
It is a compact heuristic, not a full database, and it runs in your browser.
Recent Windows sends the same version token for 10 and 11, so they cannot be told apart from the User-Agent alone.
It is a guess from keywords like Mobile and Tablet. Most cases are right, but spoofed or unusual strings can fool it.
It knows the common ones. Niche browsers often report themselves as Chrome or Safari, so that is what you will see.
No. The tool runs entirely on this page.