Best memory for 2020 iMac (27-inch): upgrade your RAM today | iMore
Memory Upgrades for iMac w/ Retina 5K Display Late 2015
OWC 64GB (4 x 16GB) PC21300 DDR4 2666MHz SO-DIMMs Memory Compatible with Mac Mini 2018, iMac 2019 and up, and Compatible PCs (OWC2666DDR4S64S) at Amazon.com
Third-Party RAM for 27-inch iMac Still Far More Affordable Than Apple's Checkout Upgrade Options - MacRumors
Install memory in an iMac - Apple Support
Third-Party RAM for 27-inch iMac Still Far More Affordable Than Apple's Checkout Upgrade Options - MacRumors