The layers of a map have a tree data structure. The root of this tree is the layer tree of the map. You can add either layers or layer groups to the layer tree.

A layer group (view/LayerGroup) can hold a collection of layers or other layer groups. In this way, you can group layers in several levels.

Program: Organizing layers in a new LayerGroup and adding it to the map. (from samples/layercontrol/main.ts) 
const backgroundGroup = new LayerGroup({
  label: "Background"
});
const world = createWorldLayer();
backgroundGroup.addChild(world, "top");

createFusionBackgroundLayer().then(function(earth) {
  backgroundGroup.addChild(earth, "below", world);
});

map.layerTree.addChild(backgroundGroup, "bottom");

To inspect the structure of the tree, use the methods and properties of view/LayerTreeNode. All layers and layer groups implement them. The children property returns a list of children of the node. You can use the accept method to traverse the tree using a view/LayerTreeVisitor.

layercontrol
Figure 1. Layers organized in several levels.