I finished all the work and I can read the data normally in debug mode. But uploading to TestFlight I don't know where the downloaded data goes?
In debug mode, the data is in the Runner.app/OnDemandResources/com.xxx.FontsPack-xxxx.assetpack directory, but the TestFlight version does not have this directory, so I I can't read the data.
So where should I go to find the downloaded Tag resource file?
You may have missed the point of how on demand resources work. If you're somehow dumpster diving to find the downloaded resources in their real location, directly, you've been doing it wrong. You just ask, in your app code, for a resource as if it were present right in your app's main bundle. If the app is downloaded and made available for use by means of a bundle resources request, it will appear to be in the main bundle and this will work.