Neat, I didn't realize it was still included. I thought it had been abandonned.
So basically one needs a webdav server somewhere or an usb flash drive.
I use local seedvault backup and then sync via round sync daily trigger to my Nextcloud WebDav Server (native seedvault was not able to use this, for some reason).
I use Seedvault to create a backup locally on my phone, and then sync it to my backup server with Syncthing
The problem is that most apps opt out of backup, so it's effectively useless.