A-Tech 8GB Kit (2x4GB) RAM for Apple MacBook Pro (Mid 2012), iMac (Late 2012, Early/Late 2013, Late 2014, Mid 2015), Mac Mini (Late 2012) | DDR3 1600MHz PC3-12800 SODIMM 204-Pin Memory Upgrade at Amazon.com
HOW I UPGRADED MY iMAC 21.5in Late 2013 MODEL (SSD & RAM UPGRADE) - YouTube
21.5" Inch iMac Late 2013 A1418 RAM PCIe NVMe SSD Hard Drive Upgrade PSU Logicboard Replacement - YouTube
Late 2013 iMac review: Faster than before, but the gains over previous models are modest | Macworld
OWC 16GB (2 x 8GB) PC21300 DDR4 2666MHz SO-DIMMs Memory Compatible with Mac Mini 2018, iMac 2019 and up, and Compatible PCs (OWC2666DDR4S16P) at Amazon.com