Ceph isn't a filesystem, it's a service layer (self-described "storage platform") that runs on top of some other unspecified filesystem. Think git-annex or hadoop, not ext4.

Anyway the way Ceph does that is replication, just like those other solutions. There may be 4 nodes with filesystems that contain that data, and Ceph is the veneer that lets you not have to worry about the implementation-detail of where it lives.