Opera still hosts very old versions of the browser on their mobile site, though these are typically for older Windows Mobile
A .XAP file is the application package format used by Windows Phone 8.1 and Windows 10 Mobile. To install an app using this format today, you cannot simply click "Install" from a web browser. You must "sideload" the application. How to Sideload Opera Mini on Windows 10 Mobile opera mini xap for windows 10 mobile link