Teardown of new 21.5-inch iMac surprisingly reveals user-upgradable RAM, modular CPU - 9to5Mac
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
Install memory in an iMac - Apple Support
Can you upgrade the RAM in the 27-inch iMac? | iMore
How to upgrade RAM in the (Mid 2017) 5K iMac and save up to ~$900 [Video] - 9to5Mac
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
Install memory in an iMac - Apple Support
A-Tech 32GB (2x16GB) RAM for Apple iMac 2017 (27 inch Retina 5K & 21.5 inch Retina 4K) | DDR4 2400MHz SODIMM PC4-19200 2Rx8 1.2V 260-Pin SO-DIMM Memory Upgrade Kit at Amazon.com
Teardown of New 4K 21.5-inch iMac Reveals Removable RAM and Modular CPU - MacRumors