This is the default storage provider and it is based on chart https://github.com/helm/charts/tree/master/stable/nfs-server-provisioner which is based on https://github.com/kubernetes-incubator/external-storage/tree/nfs-provisioner-v2.2.1-k8s1.12/nfs. It uses xfs quota to limit size of user directories (https://github.com/kubernetes-retired/external-storage/blob/nfs-provisioner-v2.2.1-k8s1.12/nfs/docs/deployment.md#arguments).
backup is a daily job to backup GESIS Hub, GESIS Binder and Gallery databases and
user and grafana persistent volumes.
pv.yaml is to create a hostPath type PV for nfs provisioner. There nfs creates its shares.