Can you upgrade the RAM in the 27-inch iMac? | iMore
Install memory in an iMac - Apple Support
Teardown of New 4K 21.5-inch iMac Reveals Removable RAM and Modular CPU - MacRumors
OWC SSD Upgrade Kits for 21.5-Inch iMac (2013 - 2019)
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
2017 21.5-inch 4K iMac memory upgrade kit includes 32GB of RAM, tools to take it apart | AppleInsider
A-Tech 16GB (2x8GB) RAM for Apple iMac 2017 (27 inch Retina 5K & 21.5 inch Retina 4K) | DDR4 2400MHz SODIMM PC4-19200 1.2V 260-Pin SO-DIMM Memory Upgrade Kit at Amazon.com