The clue with Unraid is in the name. The goal was all about having a fileserver with many of the benefits of RAID, but without actually using RAID.

For this purpose, Fuse is a virtual filesystem which brings together files from multiple physical disks into a single view.

Each disk in an Unraid system just uses a normal single-disk filesystem on the disk itself, and Unraid distributes new files to whichever disk has space, yet to the user they are presented as a single volume (you can also see raw disk contents and manually move data between disks if you want to - the fused view and raw views are just different mounts in the filesystem)

This is how Unraid allows for easily adding new drives of any size without a rebuild, but still allows for failure of a single disk by having a parity disk - as long as the parity is at least as large as the biggest data disk.

Unraid have also now added ZFS zpool capability and as a user you have the choice over which sort of array you want - Unraid or ZFS.

Unraid is absolutely not targeted at enterprise where a full RAID makes more sense. It's targeted at home-lab type users, where the ease of operation and ability to expand over time are selling points.