karhart:
https://dlz.reaper.fm/userguide/ReaperUserGuide5974c.pdf
1.15 Enabling VST Plug-ins
There’s one further Preferences screen you will probably want to visit before you get started – VST Plug-ins. As with Audio Preferences, REAPER has several pages of Plug-in Preference screens, but these can (at first at least) be left to their default settings.
However, if you have a collection of VST and/or VSTi Plug-ins (including VST3), REAPER needs to be told where to find them. Note that VST3 plug-ins should be keptin a directory or directories separate from other plug-ins. Choose the Options, Preferences command, then select VST under the Plug-ins section. You can
use the Auto-detect button to locate your plug-ins, but if they are spread across several directories REAPER
might not find them all. In this case, you can click on the Edit button to specify the location(s) of the folder(s) where your other VST plug-ins are stored. You can leave most other settings as they are (at least for now).
MacOS uses two “built in” folders for installing shared plug-ins. These are /Library/Audio/Plug-Ins and/Users/<your account>/Library/Audio/Plug-Ins. REAPER will scan these by default:: you should,