Memory Upgrades for 2019-2020 iMac 27-Inch - Up to 128GB
OWC 128GB (2 x 64GB) PC23400 DDR4 ECC-R 2933MHz RDIMMs Memory Compatible with Mac Pro 2019 and up Models (OWC2933R6M128) : Everything Else - Amazon.com
How to Upgrade/Replace the Memory in a 21.5-inch iMac (2019) iMac19,2 - YouTube