I don't understand why in Android 11 use of `storage\emulated...\downloads` folder can read/write some files and not others?

Solution 1:

With help from Mark in the comments above this solved my problem. Since the .db extension is not recognizable, I had to use intent.setType("*/*") and validate the file and extension at the resultLauncher (i.e., ActivityResultLauncher).

https://developer.android.com/training/data-storage/shared/documents-files#open-file

private void setupImportDBButton(){
    mImportDB.setOnClickListener(v -> {
        Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT);
        intent.addCategory(Intent.CATEGORY_OPENABLE);
        intent.setType("*/*");
        resultLauncher.launch(intent);
    });
}

Included within the ActivityResultLauncher: Basics of the functionality that worked to resolve the multi-device use forced by Android 11 (API 30) requirements. The key was the use of the getContentResolver().

Uri uri = result.getData().getData();
String destinationPath = this.getDatabasePath(strDBName).getPath();

    InputStream source = this.getContentResolver().openInputStream(uri);
    OutputStream destination = this.getContentResolver().openOutputStream(Uri.fromFile(new File(destinationPath)));
    DatabaseManager.copyDatabase(source, destination);