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The .wxs files are authored by you, with your requirements for the installer. You then pass those files to candle to generate the object files, and pass the object files to light to build the MSI.
If you need to edit the .wxs files, they are actually XML so the Edit XML / Merge XML /etc actions in FB will be able to work with them. Type “XML” into the Action Types filter to see all of the available XML actions.