public CIMRenderer GetRenderer()

var lyr = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault(f => f.ShapeType == esriGeometryType.esriGeometryPolygon);
if (lyr == null) return;
QueuedTask.Run(() =>
{
  // GetRenderer from Layer (assumes it is a unique value renderer)
  var uvRenderer = lyr.GetRenderer() as CIMUniqueValueRenderer;
  if (uvRenderer == null) return;
  //layer has STATE_NAME field
  //community sample Data\Admin\AdminSample.aprx
  string expression = "if ($view.scale > 21000000) { return $feature.STATE_NAME } else { return 'All' }";
  CIMExpressionInfo updatedExpressionInfo = new CIMExpressionInfo
  {
    Expression = expression,
    Title = "Custom" // can be any string used for UI purpose.
  };
  //set the renderer's expression
  uvRenderer.ValueExpressionInfo = updatedExpressionInfo;

  //SetRenderer on Layer
  lyr.SetRenderer(uvRenderer);
});

var lyr = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault(f => f.ShapeType == esriGeometryType.esriGeometryPolygon);
if (lyr == null) return;
QueuedTask.Run(() =>
{
  //Get the layer's definition
  //community sample Data\Admin\AdminSample.aprx
  var lyrDefn = lyr.GetDefinition() as CIMFeatureLayer;
  if (lyrDefn == null) return;
  //Get the label classes - we need the first one
  var listLabelClasses = lyrDefn.LabelClasses.ToList();
  var theLabelClass = listLabelClasses.FirstOrDefault();
  //set the label class Expression to use the Arcade expression
  theLabelClass.Expression = "return $feature.STATE_NAME + TextFormatting.NewLine + $feature.POP2000;";
  //Set the label definition back to the layer.
  lyr.SetDefinition(lyrDefn);
});

public Task SetFeatureLayerSymbolAsync(FeatureLayer ftrLayer, CIMSymbol symbolToApply)
{
  if (ftrLayer == null || symbolToApply == null)
    throw new System.ArgumentNullException();

  return QueuedTask.Run(() =>
  {

    //Get simple renderer from the feature layer
    CIMSimpleRenderer currentRenderer = ftrLayer.GetRenderer() as CIMSimpleRenderer;
    if (currentRenderer == null)
      return;

    //Set symbol's real world setting to be the same as that of the feature layer
    symbolToApply.SetRealWorldUnits(ftrLayer.UsesRealWorldSymbolSizes);

    //Update the symbol of the current simple renderer
    currentRenderer.Symbol = symbolToApply.MakeSymbolReference();
    //Update the feature layer renderer
    ftrLayer.SetRenderer(currentRenderer);
  });
}

public Task SetFeatureLayerSymbolFromStyleItemAsync(
           FeatureLayer ftrLayer, SymbolStyleItem symbolItem)
{
  if (ftrLayer == null || symbolItem == null)
    throw new System.ArgumentNullException();

  return QueuedTask.Run(() =>
  {
    //Get simple renderer from the feature layer
    CIMSimpleRenderer currentRenderer = ftrLayer.GetRenderer() as CIMSimpleRenderer;
    if (currentRenderer == null)
      return;
    //Get symbol from the SymbolStyleItem
    CIMSymbol symbol = symbolItem.Symbol;

    //Set symbol's real world setting to be the same as that of the feature layer
    symbol.SetRealWorldUnits(ftrLayer.UsesRealWorldSymbolSizes);

    //Update the symbol of the current simple renderer
    currentRenderer.Symbol = symbol.MakeSymbolReference();
    //Update the feature layer renderer
    ftrLayer.SetRenderer(currentRenderer);
  });
}

  // var map = MapView.Active.Map;
  // if (map == null)
  //        return;
  // var pointFeatureLayer =
  //       map.GetLayersAsFlattenedList()
  //          .OfType<FeatureLayer>()
  //         .Where(fl => fl.ShapeType == esriGeometryType.esriGeometryPoint);
  //   await ApplySymbolToFeatureLayerAsync(pointFeatureLayer.FirstOrDefault(), "Fire Station");

  public Task ApplySymbolToFeatureLayerAsync(FeatureLayer featureLayer, string symbolName)
  {
    return QueuedTask.Run(async () =>
    {
      //Get the ArcGIS 2D System style from the Project
      var arcGIS2DStyle =
Project.Current.GetItems<StyleProjectItem>().FirstOrDefault(s => s.Name == "ArcGIS 2D");

      //Search for the symbolName style items within the ArcGIS 2D style project item.
      var items = await QueuedTask.Run(() =>
      arcGIS2DStyle.SearchSymbols(StyleItemType.PointSymbol, symbolName));

      //Gets the CIMSymbol
      CIMSymbol symbol = items.FirstOrDefault().Symbol;

      //Get the renderer of the point feature layer
      CIMSimpleRenderer renderer = featureLayer.GetRenderer() as CIMSimpleRenderer;

      //Set symbol's real world setting to be the same as that of the feature layer
      symbol.SetRealWorldUnits(featureLayer.UsesRealWorldSymbolSizes);

      //Apply the symbol to the feature layer's current renderer
      renderer.Symbol = symbol.MakeSymbolReference();

      //Appy the renderer to the feature layer
      featureLayer.SetRenderer(renderer);
    });
  }

//Note: Call within QueuedTask.Run()
if (MapView.Active.Map == null) return;

//Get an existing Layer. This layer has a symbol you want to use in a new layer.
var lyr = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>()
      .Where(l => l.ShapeType == esriGeometryType.esriGeometryPoint).FirstOrDefault();
//This is the renderer to use in the new Layer
var renderer = lyr.GetRenderer() as CIMSimpleRenderer;
//Set the Dataconnection for the new layer
Geodatabase geodatabase = new Geodatabase(
  new FileGeodatabaseConnectionPath(new Uri(@"E:\Data\Admin\AdminData.gdb")));
FeatureClass featureClass = geodatabase.OpenDataset<FeatureClass>("Cities");
var dataConnection = featureClass.GetDataConnection();
//Create the definition for the new feature layer
var featureLayerParams = new FeatureLayerCreationParams(dataConnection)
{
  RendererDefinition = new SimpleRendererDefinition(renderer.Symbol),
  IsVisible = true,
};
//create the new layer
LayerFactory.Instance.CreateLayer<FeatureLayer>(
  featureLayerParams, MapView.Active.Map);