Used Apple iMac 21.5-inch (Aluminum) 2.7GHZ Quad Core i5 (Late 2013) ME086LL/A - GainSaver
Install memory in an iMac - Apple Support
OWC SSD Upgrade Kits for 21.5-Inch iMac (2013 - 2019)
Install memory in an iMac - Apple Support
HOW I UPGRADED MY iMAC 21.5in Late 2013 MODEL (SSD & RAM UPGRADE) - YouTube
16GB KIT 2X 8GB for Apple iMac 2.7GHz 21.5" Late 2013 2.93GHz 27" Late
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
NEMIX RAM 16GB Kit (2x8GB Upgrade) DDR3-1600 SODIMM PC3-12800 for Apple MacBook Pro Mid 2012 / iMac Late 2012 / iMac 2013, Late 2014, Mid 2015, Mac Mini Late 2012 at Amazon.com
Install memory in an iMac - Apple Support
How to upgrade RAM iMac late 2013 21.5inch - break & fix MOVIE - YouTube
iMac Intel 21.5" EMC 2638 Teardown - iFixit
CMS 32GB (4X8GB) DDR3 12800 1600MHz Non ECC SODIMM Memory Ram Upgrade Compatible with Apple®
Memory Upgrades for iMac 27-Inch (2012 - Mid 2015)
2013 iMac teardowns reveal SSD slots, soldered-in CPU in the 21.5” model | Ars Technica