a btrfs "array" in unraid is composed by individual btrfs partitions mounted as /mnt/disk1 /disk2 /disk3 and so on, then there's a daemon that makes a "unified" view at /mnt/user/, and it uses a different algorithm for parity.
There's a way to make real btrfs raid arrays, but it has been introduced very recently (1-2 years ago), and it's not the default that you create when you use the Web UI.