How to have multilayers icon show up?

Thanks @itamair, your answers are always detailed and useful, and I really appreciate it.

Since 2020, things have changed. Stamen tiles have moved to Stadia, and Hydda is not available. Here's a version which works in Nov 2023, based on the merge request in 🐛 Migration Guide for Stamen Map Tile Users Active .

After adding this to your custom module, clear cache, and remember to enable "Multilayers" in your View under Format > Leaflet Map > Settings > Leaflet Map Tiles Layer.

/**
 * Implements hook_leaflet_map_info().
 */
function custom_module_leaflet_map_info() {

  $map_info['multilayers'] = [
    'label' => 'Multilayers',
    'description' => t('Multilayers'),
    'settings' => [
      'dragging' => TRUE,
      'touchZoom' => TRUE,
      'scrollWheelZoom' => TRUE,
      'doubleClickZoom' => TRUE,
      'zoomControl' => TRUE,
      'attributionControl' => TRUE,
      'trackResize' => TRUE,
      'fadeAnimation' => TRUE,
      'zoomAnimation' => TRUE,
      'closePopupOnClick' => TRUE,
      'minZoom' => 2,
      'maxZoom' => 20,
      'zoom' => 15,
      'layerControl' => TRUE,
      'layerControlOptions' => [
        'position' => 'topright',
      ],
    ],
    'layers' => [
      'OpenTopoMap' => [
        'urlTemplate' => '//{s}.tile.opentopomap.org/{z}/{x}/{y}.png',
        'options' => [
          "minZoom" => 0,
          "maxZoom" => 18,
          "attribution" => 'Kartendaten: © <a href="https://openstreetmap.org/copyright">OpenStreetMap</a>-Mitwirkende, SRTM | Kartendarstellung: © <a href="http://opentopomap.org">OpenTopoMap</a> (<a href="https://creativecommons.org/licenses/by-sa/3.0/">CC-BY-SA</a>)',
        ],
      ],
      'Stamen Watercolor' => [
        'urlTemplate' => '//tiles.stadiamaps.com/tiles/stamen_watercolor/{z}/{x}/{y}.jpg',
          'options' => [
          "minZoom" => 0,
          "maxZoom" => 18,
          "attribution" => '&copy; <a href="https://stadiamaps.com/" target="_blank">Stadia Maps</a> <a href="https://stamen.com/" target="_blank">&copy; Stamen Design</a> &copy; <a href="https://openmaptiles.org/" target="_blank">OpenMapTiles</a> &copy; <a href="https://www.openstreetmap.org/about" target="_blank">OpenStreetMap</a> contributors',
        ],
      ],
      'Google Roads' => [
        'type' => 'google',
        'urlTemplate' => '//mt{s}.googleapis.com/vt?x={x}&y={y}&z={z}',
        'options' => [
          'attribution' => 'Map data &copy; <a href="https://googlemaps.com">Google</a>',
          'detectRetina' => FALSE,
          'subdomains' => [0, 1, 2, 3],
        ],
      ],

    ],
  ];

  return $map_info;

}