Memory

Before getting tied up with the outdated concept of "Memory used," please be aware that Apple changed the way RAM was handled several OS versions ago, maybe at OS10.10. Using the concept of "unused RAM is wasted RAM" they set the system to pre-stage unused RAM to improve speed. "Memory Used" is no longer the important RAM metric; today is it "Memory Pressure" and "Swap Used," both showing in Activity Monitor > Memory tab. Example, with only Safari, Mail, Messages, and Calendar open:

If Pressure is in the green and Swaps are zero or nearly zero, you have plenty of RAM regardless of how much shows "Used."

If you want to know how much RAM the iMac has?

go to: Apple > About This Mac > click on the Memory tab

If you want to know the iMac's usage and available RAM?

go to: Applications > Utility > Activity Monitor

see > How to use Activity Monitor on your Mac - Apple Support

or use a diagnostics tool called EtreCheck.

see > ‎EtreCheck on the Mac App Store