iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
Amazon.com: Apple iMac MK462LL/A 27-inch Desktop 5K Display 16GB Ram | 1TB Hard Drive (Renewed) : Electronics
Amazon.com: Late-2015 Apple iMac 21.5 with 4K Retina Display/3.1GHz Intel Core i5-5675R Quad-Core (21.5-inch, 8GB RAM, 1TB) (Renewed) : Electronics
iMac Intel 21.5" EMC 2638 Teardown - iFixit
Apple iMac A1418 2.7GHz I5 8GB Ram 1TB SATA Disk macOS Catalina | eBay
Amazon.com: OWC 32GB (2 x 16GB) PC19200 DDR4 2400MHz SO-DIMMs Memory Compatible with Mac Mini (Late 2018), 27" and 21.5" iMac (Mid 2017), and Compatible PCs (OWC2400DDR4S32P) : Everything Else