Were AMPSs “FST.”, “XM.”, “S3M.”, “IT.”, “669.” music composed on Amiga?
None of them were, those sound formats were all created on PC. The prefixes you mention are referring to FastTracker 1 & 2, Screamtracker, ImpulseTracker and Composer 669. These trackers are handling 16 to 32 channels and not 04 to 08 like on the Amiga. If you search better you will find some other PC tracker formats in AMP.