Here is my suggestion for you, many of your questions will be self-explanatory if you so choose. Open concepts tab and read through getting started and as well as the files under concepts tab. ESRI did an awesome job laying out many aspects of widget configurations and all associated files. This tab is seen at http://help.arcgis.com/en/webapps/flexviewer/help/index.html#/Getting_started/01m300000021000000/.