Amazon.com: A-Tech 64GB Kit (4x16GB) RAM for Apple iMac 2019 & 2020 27 inch Retina 5K | DDR4 2666 MHz SODIMM PC4-21300 / PC4-21333 260-Pin SO-DIMM Memory Upgrade : Electronics
Adamanta 16GB (1x16GB) 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
2019 5K iMac RAM Upgrade Guide - YouTube
Memory Upgrades for 2019-2020 iMac 27-Inch - Up to 128GB