Spotlight metadata importers are written by the developer of the product. E.g., Eastgate. They are part of the software. This is what DEVONthink does when you click the “Create spotlight index” option in a DEVONthink database’s property sheet. The spotlight metadata files are placed in a cache in ~/Library/Caches/Metadata. The importers themselves are .mdimporter files and can be found in various places in the file system including inside .app packages. (There’s a method for third parties to write importers, sometimes called Spotlight plugins.)
If you have DEVONthink and create the Spotlight index you will notice when you do a Spotlight search that there are documents listed in the results with Kind == DEVONthink Pro Office Document. Those are the metadata files – extension in this case is .dt2. On my machine other apps created metadata caches – MacJournal, Papers, VoodooPad, OmniFocus, etc. These are the files that FoxTrot uses when it builds its index. In that case, FoxTrot is doing nothing more than Spotlight and cannot give better answers to search queries than the data that the developer placed in their metadata cache files.