How to Upgrade/Replace the Memory in a 21.5-inch iMac (2019) iMac19,2 - YouTube
Apple iMac 2019 ,2020 27-inch (Retina 5k) OSX , Ram & SSD Upgrades - ZD Laptop Service Centre
Adamanta 64GB (2x32GB) Memory Upgrade for 2020 Apple iMac 27" w/Retina 5K Display, 2019 Apple iMac 27" w/Retina 5K Display, 2018 Apple Mac Mini DDR4 2666Mhz PC4-21300 SODIMM 2Rx8 CL19 1.2v DRAM
OWC Announces RAM Upgrades for New 27-Inch 5K iMacs - MacRumors