directory-level permissions for a MS Graph API registered app

First of all, I apologize if I have chosen the wrong tags for this issue.

I am trying to configure an app in Azure to use the MS Graph API. Up until now, I had been using permissions like Sites.Selected so that my app's credentials would have access to a specific site, but I am now thinking of narrowing the scope to a single directory (folder) and I'm running into problems, as I can't manage to read or write anything in the target folder within my site.

The process I followed was to grant full permissions to my app, specifically Sites.FullControl.All, and also add the Files.ReadWrite.AppFolder permission. Then, using Postman, I followed the same process as I would for granting permissions with Sites.Selected, using the ID of the folder I'm interested in and adding /permissions to the (POST) request, including something similar to this in the body:

{
    "roles": [
        "write"
    ],
    "grantedToIdentities": [
        {
            "application": {
                "id": "appid",
                "displayName": "appname"
            }
        }
    ]
}

However, it hasn't worked. Do you know what might be going wrong, or if there is another way to proceed?

I would like to emphasize that I am not an expert in either Azure or development.

Thank you in advance and best regards!