Geany loads plugins from a number of default locations (on Linux `<geany-configdir>/plugins`, `<geany-libdir>/geany`), but also you can specify a custom one in Geany Preferences.
The default Geany plugins (as packaged with Geany app) are installed in `/usr/lib/x86_64-linux-gnu/geany/`, which is basically per the distribution specific `<geany-libdir>` (in this case it's Ubuntu).