The most basic apps still are helpful because they all expose the universal Make an App module and take care of authentication.
Apps are not open sourced, so there is no way of building on top of existing apps. You’ll have to build a new Custom App from scratch. But since the app is basic and only expose the most simple endpoints that their host API exposes, it is not much work.
Building an app is straightforward until it’s not though. I’ve started this with another app. Would be so much easier to be able to reutilise work already done.