64GB RAM upgrade kit for 21.5-inch iMAC now offered by OWC - 9to5Mac
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Amazon.com: OWC 32GB (2 x 16GB) PC19200 DDR4 2400MHz SO-DIMMs Memory Compatible with Mac Mini (Late 2018), 27" and 21.5" iMac (Mid 2017), and Compatible PCs (OWC2400DDR4S32P) : Everything Else
How to Upgrade iMac RAM: 2012-2022: EveryMac.com
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Surprise! The iMac Pro has Upgradable RAM and SSD - The Mac Observer
Apple's New 21.5 Inch iMac Has Upgradeable Parts — That's a Huge Deal
Memory Upgrades for iMac w/ Retina 5K Display Late 2015
Install memory in an iMac - Apple Support
Apple iMac 2017 (mid) 21.5-inch SSD ,Ram & OSX Upgrades - ZD Laptop Service Centre