On Xcode Product menu -> Build
See [project root]/Builds/MacOSX/build/Debug/ Copy created .component file to /Library/Audio/Plug-Ins/Components/ Copy created .vst file to /Library/Audio/Plug-Ins/VST/
On GarageBand Display menu -> smart control -> master tab. -> new plugin -> see this project
https://juce.com/
https://qiita.com/COx2/items/430e07bc1e65d1485868