I was trying to help colleagues figure out how to make their middle mouse button paste on their Macs (like they used to do on Linux), and I ran into a challenge. I connected a random USB mouse that I had laying around, and when I clicked the middle button it activated the Application Switcher (like cmd+tab). I scoured the System Settings, but could not find any way to change what that button was doing. Some searching eventually led me to believe that this was a vestigial setting from a long time ago when I used the Apple Mighty Mouse. I happen to still have that mouse, and once I found it and connected it, System Settings recognized it and provided me with a "Might Mouse Settings..." button in the Mouse settings. In there I was able to change the scroll wheel press to be "Button 3", and then the other USB mouse behaved as expected (i.e., middle button didn't do anything). If I hadn't had that mouse from like 15 years ago, I'd probably still be searching for some plist file deep down in MacOS. Once the mouse behaved as expected, I was able to make a Keyboard Maestro macro to paste when Button 3 is pressed, solving the original problem.