Factory Original 64GB (2x32GB) Memory Upgrade for 2020 & 2019 Apple iMac 27" w/Retina 5K Display & 2018 Apple Mac Mini DDR4 2666Mhz PC4-21300 SODIMM 2Rx8 CL19 1.2v RAM Adamanta at Amazon.com
2019 5K iMac RAM Upgrade Guide - YouTube
How to Upgrade/Replace the Memory in a 21.5-inch iMac (2019)
Install memory in an iMac - Apple Support
Memory Upgrades for 2019-2020 iMac 27-Inch - Up to 128GB