Basic question about app / managed user

Hello, I am developing a web app with box as storage for a enterprise. I am having some problem with some basic understanding:

1. User uploads a file to the web app, which in turn uploaded to box via EnterpriseAPI
2. User has managed account by the enterprise and desktop sync installed.
3. How can user see the file he uploaded to the web app in desktop sync? (Add this user's managed account to the app's root folder?)
4. How can user drag a file into desktop sync folder and make it appear in the web app? (Via stream or webhook?)

Many thanks

Follow up, when I try to add a managed user as a collaborator, it failed with this error message

{"type":"error","status":403,"code":"access_denied_insufficient_permissions","help_url":"http:\/\/developers.box.com\/docs\/#errors","message":"Access denied - insufficient permission","request_id":"***number removed for privacy***359f404444f80c"}

My code is like this

        BoxAPIConnection api = utils.getEnterpriseBoxAPI();
        BoxFolder root = BoxFolder.getRootFolder(api);
        Iterable users = BoxUser.getAllEnterpriseUsers(api);
        users.forEach(user -> {
            BoxCollaborator user1 = new BoxUser(api, user.getID());
            root.collaborate(user1, BoxCollaboration.Role.VIEWER_UPLOADER);
        });