【3.5 Inch Hard Drive Enclosure】- The 3.5 hard drive enclosure is also available for 2.5 SATA SSD HDD enclosure. This versatility allows you to use a wide range of hard disks with the same ...

(Hard Disk Drive cradle ... Also called an "HDD caddy," "drive enclosure" or "bay adapter," such devices are widely used for quick replacement in a multi-drive RAID configuration.

The enclosure that he’s using is meant for a 5.25″ optical drive. It comes with a USB to SATA converter which is how he connects the hard drive to the Raspberry Pi serving the files.