Migrate media with alternative text
field_media_image/alt belongs in the immagini migration, not in the node migration.

In migration that creates the media item (destination plugin entity:media) you can map the media item's fields:

  # Image field see media_entity.bundle.image.yml.
  field_media_image/target_id: fid
  field_media_image/alt: field_file_image_alt_text/0/value
  field_media_image/title: field_file_image_title_text/0/value

  # Description field.
  field_description: field_image_description
  # Caption field.
  field_caption: field_caption

destination:
  plugin: entity:media


See https://www.previousnext.com.au/blog/migrating-drupal-7-file-entities-drupal-8-media-entities