Demo content, used in OpenY to present all components for the initial installation from scratch.
Migration API from Drupal 8/9 is hardly used in order to deliver demo content to all Open Y components.
Main source plugin for Demo content - embedded_data
When you need to migrate any Drupal/non-Drupal site to Open Y - demo content is 80% of the migration you need to use as a starting point. Only source plugin should be written in order to pull information from external database of source site.