The .swf extension on a file means that it was created to be played with the Adobe Flash Player - which you should NOT have on your computer anymore.
To find the .swf files on your computer, open Finder and type “swf” in the search field in the upper right of the Finder window, and be sure to click on “Name matches: swf” . Then be prepared to be amazed at how many that you have on your computer.
You should NOT be concerned with any file that is found in your Library folder, or files that are associated with an app (like Quicken), or that are associated with training videos, or that were created by an app (like JAlbum).
To know where any one of those files are located on your computer, go to the Menu Bar of the Finder app, click on View, then click on "Show Path Bar.” It will show up at the bottom of Finder. When you click on a file, the full path to the file is shown there.
Some SWF files can be viewed with QuickTime Player, some with MPV Player. But for those that you really do want to keep (or try to keep), what you really should do is to convert those files to a .mp4 or to a .mov format. QuickTime Player does have a function to export to a .mov (it doesn’t give you a choice). MPV Player does not have any function to export a movie to a different format than it opens with.
There are a few apps in the App Store to view or convert SWF files. The free one named “OmniPlayer” does NOT open any of the SWF files that I have (despite what it claims to do), and the companion app “OmniConverter” also does not recognize the .swf file. (So much for truth in advertising.) All the rest have a price tag, and I’m not willing to take a chance on a paid version to find out that it doesn’t work with all .swf files.
There are several paid versions of a converter on the Internet.
There is a free online converter, “clohttps://cloudconvert.com/swf-to-mp4,” which does work for some files. It gives several options for making the conversion, and the default works fine. It will accept multiple files at a time. It does not allow for drag and drop.
Some files may have the same name but reside in different locations. All converted files will wind up in your Download folder. So you do have to pay attention to where a particular converted file has to be moved to in order to replace an existing swf file.
So, I think your only option is to convert those that do work with QuickTime Player or can be opened with “cloud convert" and toss out the ones that can’t be viewed by QuickTime Player or converted (leaving the ones alone that are associated with an app or were created by an app for use by that app).
For any SWF file that is associated with an app, you just have to hope that an update to the app will come with non-swf files.
John R Carter Sr