I don’t think the idea has to be implemented in this way though. The key difference in my mind is that instead of having each app be its own isolated thing, you treat them more as services with APIs. One way this can be handled at the UI toolkit in the OS that could generate a something like a JSON API for each app based on the UI to query the app. This way apps could be trivially composed into custom UIs and workflows, or even scripted. This would follow the whole Unix philosophy where you have a bunch of utils that can be piped together to produce whatever functionality you need.
I don’t think the idea has to be implemented in this way though. The key difference in my mind is that instead of having each app be its own isolated thing, you treat them more as services with APIs. One way this can be handled at the UI toolkit in the OS that could generate a something like a JSON API for each app based on the UI to query the app. This way apps could be trivially composed into custom UIs and workflows, or even scripted. This would follow the whole Unix philosophy where you have a bunch of utils that can be piped together to produce whatever functionality you need.