Timetec Hynix IC 32GB KIT(2x16GB) Compatible for Apple 2019 iMac 27-inch w/Retina 5K Display, Late 2018 Mac Mini DDR4 2666MHz PC4-21300 2Rx8 CL19 1.2V SODIMM Memory RAM Upgrade (32GB KIT(2x16GB)) - Walmart.com
64GB RAM upgrade kit for 21.5-inch iMAC now offered by OWC - 9to5Mac
32GB Memory Upgrade for 2019-2020 iMac and 2018 Mac mini
NEMIX RAM Compatible for 128GB (4x32GB) DDR4 2666MHz PC4-21300 SODIMM Memory Upgrade for Apple iMac Mid 2020 (iMac 20,1/20,2) / Mid 2019 (iMac19,1) 27-inch w/Retina 5K, Late 2018 Mac Mini 8,1 at Amazon.com
How to upgrade Mac mini (2018) RAM and save money [Video] - 9to5Mac
Install memory in an iMac - Apple Support
How to Upgrade the Memory in the Late 2017 iMac Pro
Mac mini Late 2018 Memory (RAM) Replacement - iFixit Repair Guide