public T CreateLayer<T>( 
   LayerCreationParams layerDef,
   ILayerContainerEdit mapOrGroupLayer,
   LayerPosition position
)
where T: Layer

var flyrCreatnParam = new FeatureLayerCreationParams(new Uri(@"c:\data\world.gdb\cities"))
{
  Name = "World Cities",
  IsVisible = false,
  MinimumScale = 1000000,
  MaximumScale = 5000,
  DefinitionFilter = new CIMDefinitionFilter()
  {
    DefinitionExpression = "Population > 100000",
    Name = "More than 100k"
  },
  RendererDefinition = new SimpleRendererDefinition()
  {
    SymbolTemplate = SymbolFactory.Instance.ConstructPointSymbol(
      CIMColor.CreateRGBColor(255, 0, 0), 8, SimpleMarkerStyle.Hexagon).MakeSymbolReference()
  }
};

var featureLayer = LayerFactory.Instance.CreateLayer<FeatureLayer>(
  flyrCreatnParam, map, LayerPosition.AutoArrange);

var subtypeGroupLayerCreateParam = new SubtypeGroupLayerCreationParams
(
    new Uri(@"c:\data\SubtypeAndDomain.gdb\Fittings")
);

// Define Subtype layers
subtypeGroupLayerCreateParam.SubtypeLayers = new List<SubtypeFeatureLayerCreationParams>()
{
  //define first subtype layer with unique value renderer
  new SubtypeFeatureLayerCreationParams()
  {
    SubtypeId = 1,
    RendererDefinition = new UniqueValueRendererDefinition(new string[] { "type" })
  },

  //define second subtype layer with simple symbol renderer
  new SubtypeFeatureLayerCreationParams()
  {
    SubtypeId = 2,
    RendererDefinition = new SimpleRendererDefinition()
    {
        SymbolTemplate = SymbolFactory.Instance.ConstructPointSymbol(
          CIMColor.CreateRGBColor(255, 0, 0), 8, SimpleMarkerStyle.Hexagon).MakeSymbolReference()
    }
  }
};

// Define additional parameters
subtypeGroupLayerCreateParam.DefinitionFilter = new CIMDefinitionFilter()
{
  Name = "IsActive",
  DefinitionExpression = "Enabled = 1"
};
subtypeGroupLayerCreateParam.IsVisible = true;
subtypeGroupLayerCreateParam.MinimumScale = 50000;

SubtypeGroupLayer subtypeGroupLayer2 = LayerFactory.Instance.CreateLayer<SubtypeGroupLayer>(
              subtypeGroupLayerCreateParam, MapView.Active.Map);

var lyrDocFromLyrxFile = new LayerDocument(@"d:\data\cities.lyrx");
var cimLyrDoc = lyrDocFromLyrxFile.GetCIMLayerDocument();

//modifying its renderer symbol to red
var r = ((CIMFeatureLayer)cimLyrDoc.LayerDefinitions[0]).Renderer as CIMSimpleRenderer;
r.Symbol.Symbol.SetColor(new CIMRGBColor() { R = 255 });

//optionally save the updates out as a file
lyrDocFromLyrxFile.Save(@"c:\data\cities_red.lyrx");

//get a json representation of the layer document and you want store away...
var aJSONString = lyrDocFromLyrxFile.AsJson();

//... and load it back when needed
lyrDocFromLyrxFile.Load(aJSONString);
cimLyrDoc = lyrDocFromLyrxFile.GetCIMLayerDocument();

//create a layer and add it to a map
var lcp = new LayerCreationParams(cimLyrDoc);
var lyr = LayerFactory.Instance.CreateLayer<FeatureLayer>(lcp, map, LayerPosition.AutoArrange);