Plugins or API capability to connect multiple templates

We are currently in the process of planning an app that could use several current templates as our foundation for many solutions with some customization ( admin, navigation, calendar booking, peer to peer messaging, etc) Searching the forum is appears that you cannot merge templates into a single app. You can copy the UI but not the workflows and database. I apologize if this has been covered before but is it possible to use a plugin or API to communicate between templates through a single user sign-on. Basically, once the user signs in with their credentials the main application then communicates with what has been built into other templates without requiring the user to sign on again in the new template. I am just learning the system and if this is possible it would save a significant amount of time launch of an MVP. Thank you.

Use the app connector plugin. I can set this up for you for a small fee if needed. Should not take more than 45 minutes.

On the main app enable the Data API. The second app will have the app connector.