public class LayerDocument : Document 

//Note: Call within QueuedTask.Run()
var layerDoc = new LayerDocument(@"E:\Data\SDK\Default2DPointSymbols.lyrx");
var createParams = new LayerCreationParams(layerDoc.GetCIMLayerDocument());
LayerFactory.Instance.CreateLayer<FeatureLayer>(createParams, 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);

//Note: Run within QueuedTask.Run
//Get the Layer Document from the lyrx file
var lyrDocFromLyrxFile = new LayerDocument(layerFile);
var cimLyrDoc = lyrDocFromLyrxFile.GetCIMLayerDocument();

//Get the renderer from the layer file
var rendererFromLayerFile = ((CIMFeatureLayer)cimLyrDoc.LayerDefinitions[0]).Renderer as CIMUniqueValueRenderer;

//Apply the renderer to the feature layer
//Note: If working with a raster layer, use the SetColorizer method.
featureLayer?.SetRenderer(rendererFromLayerFile);

LayerDocument layerDocument = new LayerDocument(layer);
layerDocument.Save(@"c:\Data\MyLayerDocument.lyrx");