The reference management system brings 2 features in Nextcloud:
The link previews (also called reference widgets)
Both those features are generic and need to be extended by the Nextcloud apps.
Apps can add support for some HTTP links so previews are rendered in various places like Text documents and Talk messages.
The Smart Picker is a frontend component which allows users to search or generate links or text.
Apps can register Smart Picker providers to extend its capabilities. Administrators can choose which Smart Picker providers they want to make available to the users by choosing which apps they install. All the Smart Picker providers shipped in the recommended apps do not send any data to 3rd party services. Some community apps Smart Picker providers might rely on 3rd party services.