Apple's New 21.5 Inch iMac Has Upgradeable Parts — That's a Huge Deal
Install memory in an iMac - Apple Support
Adamanta 16GB (2x8GB) 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
How to Upgrade/Install Memory in a 21.5-inch iMac (Mid 2017) iMac18,1 iMac18,2 - YouTube
Install memory in an iMac - Apple Support
How to Install Up to 32GB of OWC Memory Into a 21.5-inch iMac
8GB Apple iMac 2017 21.5" Non-Retina RAM Upgrade iMac18,1