Synology makes a few excellent presentations describing the DSM (the OS of the NAS enclosures) and the Synology Hybrid RAID (SHR). SHR allows you to maximize usage of various sized disks in a RAID array and includes data redundancy. 1 drive can fail and the RAID array continues to operate without fail.
DSM videos
Part 1
Part 2
Synology Hybrid Array webinar