How can I properly download a file using Google Drive API?

Question

I'm having issues trying to get the download link of a file using Google Drive API. It says the following:

thrown in ...\google-api\src\Http\REST.php on line 134

Fatal error: Uncaught Google\Service\Exception: {
  "error": {
    "code": 403,
    "message": "The user does not have sufficient permissions for this file.",
    "errors": [
      {
        "message": "The user does not have sufficient permissions for this file.",
        "domain": "global",
        "reason": "insufficientFilePermissions"
      }
    ]
  }
}

Function:

/**
 * Downloads a file using its URL
 * @param  string $fileId
 * @return string link to download the file.
 */
public function downloadFileDriveServer(string $fileId) {
    $this->setPermissionPublic($fileId);

    /**
     * @var \Google\Service\Drive\DriveFile $file
     */
    $file = $this->service->files->get(
        $fileId, 
        array('fields' => 'webContentLink')
    );
    
    return $file->getWebContentLink();
}

Inner function:

/**
 * Tries to set the file permissions to public.
 * @param  string $fileId id of the file.
 * @return void 
 */
private function setPermissionPublic(string $fileId) {
    $permission = new \Google\Service\Drive\Permission();
    $permission->setRole('reader');
    $permission->setType('anyone');
    $this->service->permissions->create($fileId, $permission);
}

How it's beeing called:

echo $fileController->downloadFileDriveServer($fileId);

What I've tried: Change the local and the cloud scopes.

My local (in code) Scope: https://www.googleapis.com/auth/drive.

const SCOPE = \Google\Service\Drive::DRIVE;

Inside Google Cloud console, I have basically every scope of Google Drive like:

• ./auth/drive
• ./auth/drive.metadata
• ./auth/drive.metadata.readonly
• ./auth/drive.readonly

The project is internal use only and it's working for listing files, creating directories and deleting files.

Might be relevant: I'm not using frameworks and I've seen some questions right here and I don't understand yet where to use a:

GET https://www.googleapis.com/drive/v3/files/fileId?alt=media

I know what a GET is, but don't know where to use this "GET" line above.

Any support to enable my files to be downloaded would be appreciated. Thanks a lot.