Memory Upgrades for iMac w/ Retina 5K Display Late 2015
M1 Mac RAM and storage can be upgraded after purchase, but it's not easy - 9to5Mac
A-Tech 16GB (2x8GB) RAM for Apple MacBook Pro (Early/Late 2011), iMac (Mid 2010 27 inch 4-Core, Mid 2011 21.5/27 inch), Mac Mini (Mid 2011) | DDR3 1333MHz PC3-10600 204-Pin SODIMM Memory Upgrade Kit at Amazon.com
Amazon.com: 2020 Apple iMac with Retina 5K Display (27-inch, 8GB RAM, 256GB SSD Storage) : Electronics