In Android 11 the space for an app is 'private' - only accessible for the app and not for other apps like editors.
I see 2 ways for you:
- use a Windows (or other system) editor. Copy brf via USB to Windows, edit it and copy back. Usful at home.
- the remote way. Apps can send a remote profile to the BRouter app. So you could store your brf in an editable folder (like Download). But I can't say if osmand can do that for you.