public void SetSnappable( 
   bool isSnappable
)

var featureLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault();
// is the layer snappable?
bool isSnappable = featureLayer.IsSnappable;
// set snappability for a specific layer - needs to run on the MCT
await QueuedTask.Run(() =>
{
  // use an extension method
  featureLayer.SetSnappable(true);

  // or use the CIM directly
  //var layerDef = fLayer.GetDefinition() as ArcGIS.Core.CIM.CIMGeoFeatureLayerBase;
  //layerDef.Snappable = true;
  //fLayer.SetDefinition(layerDef);
});
// turn all layers snappability off
var layerList = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>();
await QueuedTask.Run(() =>
{
  foreach (var layer in layerList)
  {
    layer.SetSnappable(false);
  }
});

// interested in only snapping to the vertices of a specific layer of interest and not the vertices of other layers
//  all other snapModes should be off.

// snapping must be on
Snapping.IsEnabled = true;

// turn all application snapModes off
Snapping.SetSnapModes(null);

// set application snapMode vertex on 
Snapping.SetSnapMode(SnapMode.Vertex, true);

// ensure layer snapping is on
await QueuedTask.Run(() =>
{
  featureLayer.SetSnappable(true);
});

// set vertex snapping only
var vertexOnly = new LayerSnapModes(false) { Vertex = true };

// set vertex only for the specific layer, clearing all others
var dict = new Dictionary<Layer, LayerSnapModes>();
dict.Add(featureLayer, vertexOnly);
Snapping.SetLayerSnapModes(dict, true);  // true = reset other layers

{
    if (!layer.IsVisible)
        layer.SetVisibility(true);

    if (layer is FeatureLayer featureLayerToChange)
    {
        if (!featureLayer.IsEditable)
            featureLayer.SetEditable(true);

        if (!featureLayer.IsSnappable)
            featureLayer.SetSnappable(true);
    }
}