How to share some content in a multisite?
There are quite a few solutions available in Drupal contrib which allows sites to share the content with each other:


Workspace In core now creates multiple versions of the site.
Entity Pilot is a really good use core serializer solution. It tracks changes allow and merging via UI. If you want to track the content then you can also use Entity Pilot Git
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CPS D7 version only.
Domain access if your site shares the domain name then this is the best solution to display the same content on different domains.
Default Content you have to manually import export the content.
D8 to D8 migration is the last ditch solution there is no D8 to D8 migration in Drupal core at this moment but you can export content in CSV using CSV Serialization and then import it via Migrate Source CSV. The sites can periodically import or export data in hook_cron.