How to: 27-inch 5K iMac (2019) 128GB RAM Upgrade - Save $$$! - YouTube
Adamanta 16GB (2x8GB) 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 1Rx8 CL19 1.2v DRAM
Timetec Hynix IC 16GB Compatible for Apple 2019 iMac 27-inch w/Retina 5K Display, Late 2018 Mac Mini DDR4 2666MHz PC4-21300 2Rx8 CL19 1.2V SODIMM Memory RAM Upgrade (16GB) - Newegg.com
Amazon.com: A-Tech 32GB Kit (2x16GB) RAM for Apple iMac & Mac Mini (2018, 2019 & 2020) | DDR4 2666 MHz SODIMM PC4-21300 / PC4-21333 260-Pin SO-DIMM Memory Upgrade : Electronics
Tech 101: Order the Right Amount of Memory for Your New Mac