System Preferences can be extended with plugins.
Plugins inherit PreferencesModulePlugin and return a new PreferencesModule instance that has preflet information such as name, description, icon name, category, whether it requires administrative privileges (helpful for preflets that deal with system settings).
System Settings has a simple model that iterates over the plugins directory, load plugins with QPluginLoader and create module instances.
In the QtWidgets-based version, PreferencesModule inherited from QWidget, the preflet was then a widget that System Preferences added to a stacked widget. When the corresponding icon was clicked, the current index was changed showing the preflet's widget.
Since I'm rewriting everything in QML, PreferencesModule now inherits from QObject and has a new item() method that preflets implement returning a QQuickItem object created by loading the QML code embedded into the resources with QQmlComponent.
The loading mechanism is pretty much the same, the model exposes the QQuickItem with the item role, when the preflet icon is clicked the item is pushed to a StackView.