Adamanta 8GB (1x8GB) Memory Upgrade Compatible for 2017 Apple iMac 27"/21.5" Retina 5K/4K or Non-Retina Display DDR4 2400MHz PC4-19200 SODIMM 1Rx8 CL17 1.2v Single Rank RAM DRAM at Amazon.com
Apple iMac (27-inch, 2017) review: Apple doesn't mess with success - CNET
How to Upgrade/Install Memory in a 21.5-inch iMac (Mid 2017) iMac18,1 iMac18,2 - YouTube