code: def fill_layer_combo(self):
          
          project = QgsProject.instance()
          # MapLayers returns a QMap<QString id, QgsMapLayer layer>
          layers = list(project.mapLayers().values())
  
          extensions = tuple(extension_siblings.keys())
          for layer in layers:
              if layer.source().lower().endswith(extensions):
                  icon = layer_icon(layer)
                  self.layers.addItem(icon, layer.name(), layer.id())
docs: Fill layer combobox.