How to Upgrade/Replace the PCIe SSD in a 21.5-inch iMac (Late 2013) iMac14,1 iMac14,3 - YouTube
Apple iMac 21.5” SSD Upgrade (2013-2015) HDD To Solid State Drive (SATA) | Step By Step - YouTube
OWC SSD Upgrade Kits for 21.5-Inch iMac (2013 - 2019)
SSD for iMac 21.5-Inch (Late 2013 - Current) - ynzal.com
Restored Apple Desktop Computer iMac 21.5-inch (Aluminum) 2.9GHZ Quad Core i5 (Late 2013) ME087LL/A 8 GB DDR3 1 TB HDD 1920 x 1080 Display Sierra 10.12 Includes Keyboard and Mouse (Refurbished) - Walmart.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