Crucial Memory Bundle with 16GB (2 x 8GB) DDR4 2666MHz SODIMM Upgrade kit CT2K8G4SFS8266 Compatible with iMac Retina 5K 27-inch 2019 (iMac19,1) and Mac Mini 2018 (Macmini8,1) at Amazon.com
Amazon.com: Crucial RAM 8GB DDR4 2666 MHz CL19 Memory for Mac CT8G4S266M : Everything Else
Memory Upgrades for 2019-2020 iMac 27-Inch - Up to 128GB
Install memory in an iMac - Apple Support
How to Install Ram on the 2019 27 inch iMac and which Ram to Buy? - YouTube