Perfectly legit question, so don't worry about sounding noobish
C3 plugins can specify that they require cordova plugins. For instance the inbuilt Google Play plugin includes the construct-play-games cordova plugin.
When you export as cordova project it will add the "construct-play-games" plugin to the "config.xml" file. If your not familiar with that it's the main project file for cordova projects, so it holds all the project settings.
When you export as an Android studio project, a debug APK, a release APK or a signed APK the build service checks the projects cordova plugins against a list of known safe ones and includes them into the build.
If your adding cordova plugins to the config.xml file manually before uploading to phonegap build or similar then we don't currently support that. Although we have been considering adding an option for including additional cordova plugins in your build.
If your interested to adding cordova plugins to a C3 plugin checkout AddCordovaPluginReference on the specifying dependencies manual page.