Amazon.com: Apple Early 2019 iMac Retina 5K with 3.0GHz Intel Core i5 (27-inch, 16GB RAM, 512GB Storage) - Silver (Renewed) : Electronics
Adamanta 64GB (2x32GB) Memory Upgrade for 2020 Apple iMac 27" w/Retina 5K Display, 2019 Apple iMac 27" w/Retina 5K Display, 2018 Apple Mac Mini DDR4 2666Mhz PC4-21300 SODIMM 2Rx8 CL19 1.2v DRAM
Memory Upgrades for 2019-2020 iMac 27-Inch - Up to 128GB
Install memory in an iMac - Apple Support
2019 5K iMac RAM Upgrade Guide - YouTube
How to Install Ram on the 2019 27 inch iMac and which Ram to Buy? - YouTube