Apple - iMac (21.5-inch, Mid 2011) i5, 8GB, 256GBSSD, High Sierra- Ref – Ordinateur St Joseph
How (And Why) To Install More RAM In Your iMac
Install memory in an iMac - Apple Support
Upgrading 27" iMac (2011) To 32GB Of RAM - YouTube
Cracking Open the 27" Apple iMac (2011) | TechRepublic
Amazon.com: Timetec 16GB KIT(2x8GB) Compatible for Apple DDR3 1333MHz PC3-10600 CL9 for Mac Book Pro (Early/Late 2011 13/15/17 inch), iMac(Mid 2010, Mid/Late 2011 21.5/27 inch), Mac Mini(Mid 2011) MAC RAM Upgrade :
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
Memory Upgrades for Apple iMac 2011 Up to 32GB
How To Upgrade RAM in a Mac | Macworld
A-Tech 16GB (4x4GB) RAM for Apple iMac (Mid 2010, Mid 2011, Late 2011) | DDR3 1333MHz PC3-10600 204-Pin SODIMM Memory Upgrade Kit at Amazon.com