This is the documentation for v7 but the latest version is v9. You can switch versions in the menu at the top. Check your current version with the following command:

composer show spatie/laravel-medialibrary

Downloading a single file

Media implements the Responsable interface. This means that you can just return a media object to download the associated file in your browser.

use Spatie\MediaLibrary\Models\Media;

class DownloadMediaController
{
   public function show(Media $mediaItem)
   {
      return $mediaItem;
   }
}

If you need more control you could also do the above more verbose:

use Spatie\MediaLibrary\Models\Media;

class DownloadMediaController
{
   public function show(Media $mediaItem)
   {
       return response()->download($mediaItem->getPath(), $mediaItem->file_name);
   }
}