OWC SSD Upgrade Kits for 21.5-Inch iMac (2013 - 2019)
2013 iMac teardowns reveal SSD slots, soldered-in CPU in the 21.5” model | Ars Technica
iMac Late 2013 21.5 SSD Upgrade - YouTube
Apple iMac 21.5” SSD Upgrade (2013-2015) HDD To Solid State Drive (SATA) | Step By Step - YouTube
Memory Upgrades for iMac 21.5-Inch 2012 - Late 2013
iMac | 2.7 GHz Intel i5 | 21.5 Inch | Late 2013 - Techable.com
I got a FREE 2013 iMac 21.5 with internal upgrades including Samsung evo ssd and 16gigs of ram. Then used open legacy patcher to install Mac OS Venture 13.5 beta. : r/osx
iMac 2013 (and later) with NVME SSD? | MacRumors Forums