Downloading a backup file directly from Google Drive as a standard file is not supported by Google's native interface . While you can locate the backup's presence in your storage settings, the data is encrypted and hidden from the main file list to protect user privacy.

To "download" or access this data, you must either use the official restore process on a mobile device or employ specialized third-party tools for PC extraction. Locating Your Backup on Google Drive

Before attempting a download, you can verify that a backup exists and see its last update time. Couldn't find Whatsapp Backup File - Google Drive Community