I have did quite a bit of research on this topic, but could not find any similar issues on the Internet. (But I have often overlooked well--discussed issues elsewhere). I use AI on Ubuntu and an Android 10 phone.
It appears that the location of saved files is spurious (or at least not convenient at all). The documentation says that if the filename is prefixed with a '/', then the mit ai app writes the output file to the SD card. In my case, within the Companion on Android, it is written, not to SD card but to to Main storage/Android/data/edu.mit.appinventor.aicompanion3/files/. If I build and install the the app, then the output file goes to Main storage/Android/data/appinventor.ai_myname.BatMon8/files/. This is inconvenient. Is there a way of routing file output to the SD card?