Airtable Apps, available on Pro plans, are modular apps that you can add to your bases to extend their functionality. They also look great in your Airtable Universe bases—try a map app, or a chart app!
If you have apps installed in a base that you'd like to publish, you have a choice of whether or not you want those apps to appear in Airtable Universe. At the bottom of the publish screen for your base, there is a Show apps in published base toggle you can turn on.
When someone makes a copy of your Universe base that has apps in it, those apps will persist, i.e., their copy will include copies of the apps that were in your base. If the person who copies your base is on a free or Plus plan after the initial Apps trial expires, the apps will be copied, but they will not be able to access or edit them (unless they upgrade to Pro in the future, at which point they will be able to edit or delete those apps).
Note that when the base copy and included apps are copied, API keys from apps which use them are NOT copied over. The copying person will need to enter their own API key to enable those copied apps.