[miedpo]

They do use btrfs. However, Synology also uses some additional tools on top of btrfs. From what I remember (could be wrong about the precise details), they actually run mdadm on top of btrfs, and use mdadm in order to get the erasure coding and possibly the cache NVME disk too. (By erasure coding, I mean RAID 5/6, or SHR, which are still unstable generally in BTRFS).

[janc_]

I assume you mean running btrfs on top of md (mdadm) or dm (dmraid), not the other way around?

[miedpo]

Woops, you are correct! And it looks like it is dmraid, not mdadm.

https://daltondur.st/syno_btrfs_1/

Sorry about that!