Review: 21.5-inch 2012 iMac takes two steps forward, one step back | Ars Technica
Apple iMac 21.5-inch (December 2012) review: The smaller iMac Windows PCs can pick on - CNET
Amazon.com: Hagibis iMac Hub with 1000Mbps RJ45, USB C 3.1, USB 3.0 Ports and SD/Micro SD Card Reader, USB-C Clamp Hub USB C Docking Station for 2021 iMac 24 inch (with RJ45) :
Adding USB3.0 to late 2009 iMac (11,1) | MacRumors Forums