what does this code do?
const LevelPickerApp = () => {
  const { iTwinId, iModelId, authClient } = useViewerContext();
  const uiProviders = [new LevelSelectorWidgetProvider()];

  /** Setup view state */
  const viewportOptions: ViewerViewportControlOptions = {
    viewState: async (iModelConnection) => {
      const viewState = await ViewSetup.getDefaultView(iModelConnection);
      return viewState;
    },
  };

  const onIModelAppInit = async () => {
    IModelApp.viewManager.onViewOpen.addOnce(ITwinLink.initializeLink);
    IModelApp.viewManager.onViewOpen.addOnce(async () => {
      await ITwinLink.getLevels();
    })

  }

  return authClient &&
    <Viewer
      iTwinId={iTwinId}
      iModelId={iModelId}
      authClient={authClient}
      viewportOptions={viewportOptions}
      defaultUiConfig={default3DSandboxUi}
      mapLayerOptions={mapLayerOptions}
      onIModelConnected={onIModelAppInit}
      enablePerformanceMonitors={false}
      uiProviders={uiProviders}
      theme="dark"
    />;
};
