public IReadOnlyList<Layer> GetLayersAsFlattenedList()

{
    List<FeatureLayer> featureLayerList = map.GetLayersAsFlattenedList().OfType<FeatureLayer>().ToList();
}

{
    //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();
}

{
    //Finds layers by name and returns a read only list of Layers
    IReadOnlyList<Layer> layers = map.FindLayers("cities", true);

    //Finds a layer using a URI.
    //The Layer URI you pass in helps you search for a specific layer in a map
    var lyrFindLayer = MapView.Active.Map.FindLayer("CIMPATH=map/u_s__states__generalized_.xml");

    //This returns a collection of layers of the "name" specified. You can use any Linq expression to query the collection.  
    var lyrExists = MapView.Active.Map.GetLayersAsFlattenedList()
                       .OfType<FeatureLayer>().Any(f => f.Name == "U.S. States (Generalized)");
}

{
    IEnumerable<Layer> matches = map.GetLayersAsFlattenedList().Where(l => l.Name.Contains("partialLayerName", StringComparison.CurrentCultureIgnoreCase));
    List<Layer> layers = new List<Layer>();
    foreach (Layer l in matches)
        layers.Add(l);
    System.Diagnostics.Debug.WriteLine($"Found {layers.Count} layers with name containing '{"partialLayerName"}'");
}