This is a Web App and there is no "Backend". All data and files will stay on your device. The conversion is solely done by your computers resources.
Just click on the button to upload the files you want to convert, select the files and pick them up from your download directory.
Due to the nature of software development, and that I'm not related to Garmin in any way, it might happen, that future Xero Firmwares or Shotview versions will produce incompatible files. If you have created files with a newer version, or your file just doesn't work, just let me know on Github: Report an issue.