If you are self hosted, you can store backups locally and copy them elsewhere via scp/rsync.
You can use many s3 compatible services. Configure an S3 compatible object storage provider for uploads
The point of s3 is that it’s reliable and they beck it up so you don’t have to.
If you’re hosted at discourse.org then you don’t need to download backups since they handle that for you.
Why do you want download the backups?