How to upgrade the memory of a 27-inch iMac with Retina 5K display (Late 2014 - Mid 2015) iMac15,1 - YouTube
Apple iMac 27" Inch Desktop Intel Core i7 3.7 GHz 16GB RAM 1TB HDD 2012 Catalina | eBay
iMac (27-inch, Late 2012) - Technical Specifications
A-Tech 8GB RAM for Apple MacBook Pro (Mid 2012), iMac (Late 2012, Early/Late 2013, Late 2014, Mid 2015), Mac Mini (Late 2012) | DDR3 1600MHz PC3-12800 SODIMM 204-Pin Memory Upgrade at Amazon.com
Apple iMac (Late 2012) review (21.5- and 27-inch) | Macworld