How to create menu overview? I'm creating a custom menu in the administration for my module with multiple elements. This works fine by defining the forms, routes and menu links.

How can I create a page like /admin/config for my custom menu? Is it created somehow automatically or do I really need to create a custom page that lists all submenu items in a custom controller?
You can use SystemController::systemAdminMenuBlockPage as controller in the route of parent menu of module. Some like this:

mymodule.overview:
  path: '/admin/mymodule'
  defaults:
    _controller: '\Drupal\system\Controller\SystemController::systemAdminMenuBlockPage'
    _title: 'My module overview'