When the Map Layers are open in the Information panel, you can quickly toggle between the map legend and the Map Layers by clicking the button at the bottom of the panel.
When Map Layers are listed, the button displays as .
When the Map Legend is listed, the button displays as .
What you see in the Legend list depends on the map and the layers that are available, as well as what extent you are zoomed into.
Map Legend displayed in the Map Layers panel