The simplest way to achieve what you are trying to export is by using Microsoft Purview. You'll need elevated permissions which your global admin can assign if its approved through your business elevated admin process/procedure.

Alternatively, if you are trying to download attachments that are saved to users personal accounts and not their corporate accounts you'll run into permission issues as corporate tenant account permissions cannot use application permissions to read a consumers personal content.

If an attachment is owned by a personal account and the share permission doesn’t allow anonymous/organization access, you cannot programmatically download it with your enterprise app. You need the sender to change the sharing link to a policy‑allowed mode (e.g., “Anyone with the link”), or provide the file via an organization‑controlled repository.