A-Tech 16GB (2x8GB) RAM for Apple MacBook Pro (Early/Late 2011), iMac (Mid 2010 27 inch 4-Core, Mid 2011 21.5/27 inch), Mac Mini (Mid 2011) | DDR3 1333MHz PC3-10600 204-Pin SODIMM Memory Upgrade Kit at Amazon.com
Apple iMac review (21.5-inch, 2015): 4K is optional, faster hard drives shouldn't be | Engadget
Amazon.com: Apple iMac 27" with Retina 5K Display, 3.3Ghz 6-Core Intel i5, 8GB RAM, 512GB SSD, AMD Radeon Pro 5300 4GB, Mid 2020 : Electronics
Install memory in an iMac - Apple Support
Memory Upgrades for iMac w/ Retina 5K Display Late 2015