The SSD you are seeing is the one mounted on the back side on the logic board. Review this guide [guide|57835]
The Samsung 256GB Blade SSD offers a PCIe 3.0 x4 interface and nearly twice as fast as the SSDs found in the 2013 and 2014 systems.
If you want to add in a 2.5” SATA SSD into the system you will need a cradle and a SATA cable. Follow this guide to open the system [guide|57817] and here’s the needed parts:
* [https://beetstech.com/product/imac-21-5-inch-a1418-hard-drive-cradle-923-0326|HD Cradle]
* [https://beetstech.com/product/imac-21-5-inch-a1418-hard-drive-power-data-combo-cable-923-00565|SATA cable]
* [https://beetstech.com/product/lcd-display-adhesive-tape-kit-076-1416|Display Adhesive strip kit]