The dropbutton type displays a set of links as drop-down buttons. The links can be themed separately from other links by creating a links--dropbutton.html.twig file, and you can also add a #subtype attribute to make it even more specific. For example, if you add '#subtype' => 'foo', the template suggestions will go:

• links--dropbutton--foo.html.twig
• links--dropbutton.html.twig
• links.html.twig

Consider this example:

$form['test'] = array( 
  '#type' => 'dropbutton', 
  '#links' => array( 
    'action_one' => array( 
      'title' => $this->t('Action 1'), 
      'url' => new Url('<front>'), 
    ), 
    'action_two' => array( 
      'title' => $this->t('Action 2'), 
      'url' => new Url('<front>'), 
    ), 
  ), 
);