How to gain access to a specific folder and write a file into it using Scoped Storage

My app has a feature that exports GPS data in txt format into a shared folder (now accessed with getExternalStorageDirectory), and I have to switch it to Scoped Storage (API 30).
(As information, the app is the open source GPS Logger for Android.)

In the future I would let the users choose the folder to be used for the exportation, using:

public void openDirectory(Uri uriToLoad) {
    // Choose a directory using the system's file picker.
    Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT_TREE);
    startActivityForResult(intent, your-request-code);
}

public void onActivityResult(int requestCode, int resultCode,
        Intent resultData) {
    if (requestCode == your-request-code && resultCode == Activity.RESULT_OK) {
        // The result data contains a URI for the directory that the user selected.
        Uri uri = null;
        if (resultData != null) {
            uri = resultData.getData();
            // Perform operations on the document using its URI.
        }
    }
}

This way my app will gain access to the selected folder.

How can I create a text file into that folder and write data using a BufferedWriter?

I know that maybe I can use something like:

...
Uri fileUri = // Something related to the previous uri, but I cannot find the solution
OutputStream outputStream = getContentResolver().openOutputStream(fileUri, "rw");
txtBW = new BufferedWriter(new OutputStreamWriter(outputStream));
...

but I found nothing that works.

DocumentFile is Google's somewhat clunky solution for navigating through document trees. The recipe for what you want should be:

• Take the Uri that you got for your tree and wrap it in a DocumentFile using DocumentFile.fromTreeUri()
• Call createFile() on that DocumentFile, supplying a filename or other display name, which will give you a DocumentFile representing that document
• Call getUri() on the document's DocumentFile to get a Uri representing the document
• Use openOutputStream() on a ContentResolver to write your desired content to that Uri

Note that if you need long-term access to this tree, be sure to call takePersistableUriPermission() on a ContentResolver, passing in the Uri for the document tree. That should give you access to the tree and all documents inside of it.