Teardown of New 4K 21.5-inch iMac Reveals Removable RAM and Modular CPU - MacRumors
2017 21.5-inch 4K iMac memory upgrade kit includes 32GB of RAM, tools to take it apart | AppleInsider
Install memory in an iMac - Apple Support
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
OWC Confirms New Low-End 27-inch iMac Can Be Fitted With 64GB RAM - MacRumors
Memory Upgrades for 2017 iMac 27-Inch
iMac Pro Memory Maxxer RAM Upgrade Kit
Install memory in an iMac - Apple Support
The New 2017 iMac: Notes & Tidbits on RAM - The Mac Observer