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
How to Install Ram on the 2019 27 inch iMac and which Ram to Buy? - YouTube
How to UPGRADE RAM in the (Mid 2017) 5K iMac & save $$$! - YouTube