How to Upgrade/Replace the Memory in a 21.5-inch iMac (2019) iMac19,2 - YouTube
How to upgrade the 27-inch 5K iMac (2019) with a massive 128GB of RAM [Video] - 9to5Mac
A-Tech 16GB (2x8GB) RAM for Apple iMac Late 2015 27 inch Retina 5K | DDR3L 1866MHz / 1867MHz PC3L-14900 1.35V 204-Pin SODIMM Memory Upgrade Kit at Amazon.com
Timetec 32GB Compatible for Apple DDR4 2666MHz for Mid 2020 iMac (20,1 / 20,2) / Mid 2019 iMac (19,1) 27-inch w/Retina 5K Display, Late 2018 Mac Mini (8,1) PC4-21333 / PC4-21300 SODIMM MAC RAM Upgrade at Amazon.com
Apple iMac 5K 27-inch (Mid 2019) 3.6GHz i9 64GB RAM 4TB SSD - Buy Refurbished iMac 2019 at Techable
Refurbished 27-inch iMac 3.6GHz 10-core Intel Core i9 with Retina 5K display - Apple