The 27" iMac used to have upgradable RAM, but the text for the new version says "configurable to 64 GB", not "upgradable" like it used to. I'd love to be proven wrong, though, I'm considering getting an iMac.
Unless something changed with today's update, it should be upgradeable via a hatch on the back of the machine. I upgraded my 2018 model to 40GB less than a year ago.
