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
Crucial Memory Bundle with 32GB (2 x 16GB) DDR4 2666MHz SODIMM Upgrade kit CT2K16G4SFD8266 Compatible with iMac Retina 5K 27-inch 2019 (iMac19,1) and Mac Mini 2018 (Macmini8,1) at Amazon.com
Install memory in an iMac - Apple Support
How to Upgrade/Install Memory in a 21.5-inch iMac (Mid 2017) iMac18,1 iMac18,2 - YouTube
Apple 2020 iMac 27 Inch 5K 3.6GHz i9 8TB SSD 128GB RAM 16GB GFX 10GBe – Quality Macs