Thanks for your response! Very detailed.  I've tried this I believe without much luck.  I've created a small app just to test file access which you can see here:
I've configured the app to have a UC Volume:

The app is running, but I am not able to access to volume directly, only if I go through Databricks SDK. This is what I see when I try to use python OS functions:

Using Databricks SDK from another endpoint on the same app, I'm able to do it though:

So the question now is why the mount is not available to the App?

Thanks!

Anton