public void SetBasemapLayers( 
   Basemap basemap
)

{
    //Basemaps stored locally in the project. This is usually an empty collection
    //Note: Needs QueuedTask to run
    string localBasemapTypeID = "cim_map_basemap";

    var mapContainer = Project.Current.GetProjectItemContainer("Map");
    mapContainer.GetItems().Where(i => i.TypeID == localBasemapTypeID).ToList();

    //portal basemaps. If there is no current active portal, the usual default
    //is arcgis online
    var portal = ArcGISPortalManager.Current.GetActivePortal();
    var portalBaseMaps = await portal.GetBasemapsAsync();

    //use one of them...local or portal...
    //var map = MapView.Active.Map;
    //QueuedTask.Run(() => map?.SetBasemapLayers(portalBaseMaps[0]));  
}

{
    //Note: Run within QueuedTask
    map.SetBasemapLayers(Basemap.Gray);
}

{
    //Note: Run within QueuedTask
    map.SetBasemapLayers(Basemap.None);
}