drupal/core/modules/media/media.links.action.yml

media.bundle_add:
  route_name: entity.media_type.add_form
  title: 'Add media type'
  appears_on:
    - entity.media_type.collection

media.add:
  route_name: entity.media.add_page
  title: 'Add media'
  weight: 10
  appears_on:
    - entity.media.collection