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Just to clarify, Apple has no warranty problem if a user installs 3rd party RAM, assuming you don't damage the Mac doing so. Most users can do this easily. Apple owner's manuals give clear directions for doing so.
Apple RAM tends to be very expensive compared to other vendors. Since computers can be fussy about RAM, use a vendor that has a no-questions asked exchange policy. A few of the more popular vendors are Other World Computing, Micron, and Datamem. Some vendors are more Mac friendly than others in helping you choose the right RAM for your Mac; I tend to patronize them.
Jan 18, 2018 APPLE Service Manual & Repair Guide for iMac 2009 - 2011 PDF Download Guide to Mac Diagnostic and Repair PDF Download 2009 - 2011 Apple Technician Guide: iMac 21.5-inch, Mid 2011 iMac 27-inch, Mid 2011 iMac 21.5-inch, Mid 2010 iMac 27.
Imac 27 Late 2012 Specs
Late 2014 Imac 27
Further, save your old RAM. If your Mac acts up during the warranty period, try reinstalling the original RAM, then if further service is needed, leave the original RAM in so there are no questions about bad 3rd party RAM.