Duplicate content between multi-sites
On the publishing end, you'd need to categorize the data by site (e.g. use term references). Then expose that data as JSON using the built-in Views module. Allow that view to filter by site, to allow the subsites to select the content it wants to import.

On the consuming end:


There used to be the Feeds module that allowed a Drupal site to fetch data from a source and load that data onto content types on the site. But that module is still in development.
Alternatively, you could build a migration that is triggered in a hook_cron(). Migrate is already in core, I recommend also installing Migrate Plus and Migrate Tools for additional utilities.


In both cases, they would hit the main site at the url that Views is exposing, filter by the site term they want, and consume the JSON it returns.