Drupal 9: Map Multiple Migration Sources Against A Single Field

This will map the results of two different migrations from two different fields to a single field in a new entity.

process:
  field_page_ref:
    - plugin: migration_lookup
      migration:
        - event
        - news
      source_ids:
        event:
          - field_og_event_ref/0/target_id
        news:
          - field_og_news_ref/0/target_id