I assume you've checked Live's Preferences > Plug-Ins tab to make sure that the various 'Use VST2..', 'Use VST3.." and 'Use Audio Units' options are all enabled. You said you scanned -- I assume with the "Rescan Plug-Ins' button -- if that didn't work maybe if you disable and then re-enable the plug-in formats it would get Live to re-scan then more thoroughly..?
Third-party plug-ins sometimes do appear as folders [containing factory presets] in the Browser here, but when I drag them into the channel strip area [at the bottom] the plug-ins do load and run as expected. This is with regular Ableton Live and a Live demo version -- I don't know if there are any restrictions in any free/bundled Live versions regarding third-party plug-ins, though I wouldn't expect there to be..
Here's a link to Ableton's troubleshooting page regarding plug-ins:
It offers some additional notes on plug-in issues [like bit depth restrictions and compatibility with the new M1 Macs] and suggestions [like resetting Live's database]..