I wrote something about VST2 and VST3 plugins on Windows, so I thought I should add info about Mac OS. I don’t have a Mac (what!), but this is the information I have found.
The VST2 standard does not know any mandatory folder, but on Apple systems there is a defined plugin folder within the system’s folder structure since the first version of Mac OS X. All VST plugin installers for Mac are using these folders (AFAIK).
The VST2 plugins have the extension .vst and are stored in these folders:
Users/$USERNAME/Library/Audio/Plug-ins/VST (rarely used)
The newer VST3 format has a dedicated installation path all VST3 plugins must comply with.
The VST3 plugins have the extension .vst3 and are stored in these folders:
Users/$USERNAME/Library/Audio/Plug-ins/VST3 (rarely used)
VST plug-in locations on Mac OS X and macOS https://helpcenter.steinberg.de/hc/en-us/articles/115000171310
VST 3 Locations / Format https://steinbergmedia.github.io/vst3_doc/vstinterfaces/vst3loc.html