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 Seedvault to create a backup locally on my phone, and then sync it to my backup server with Syncthing
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).
The problem is that most apps opt out of backup, so it's effectively useless.