Review on 💾 8GB PC21300 DDR4 2666MHz SO-DIMM Memory: Compatible with Mac Mini 2018, iMac 2019 and newer, as well as Compatible PCs by Julio Rattanajatuphorn

I just opened the back of my new 2019 iMac 5K and threw them inside. Make sure the slots they go into are staggered, so either 1 and 3 or 2 and 4, then fill the other staggered slots with your existing RAM (if you have them). I turned on the computer for the first time and it recognized the memory and gave me 40GB, including the 8GB that came with the computer. Good stuff!