iMac (21.5-inch, 2017) How do I add more RAM??

21.5" iMacs do not have user upgradeable RAM, you can ask your local independent Apple Authorized Service Provider to see if they will do the work but many will not as it requires a complete disassembly of the computer. Apple never made the 21.5" iMacs to be worked on by end users. Please watch the following video for what it entails and you will begin to see what I mean.

https://eshop.macsales.com/installvideos/imac-pro-2017/?filter=memory

You can use /Applications/Utilities/Activity Monitor.app while you are using your typical applications to see exactly what your memory usage is, and if you really need more RAM. You want to try to avoid using large amounts of Swap, this means you lack sufficient RAM and the computer is using space on the hard drive to meet the demands. The Swap memory can be much slower and make your machine slow down when it is used a lot.

everymac.com has detailed listings of old iMac models that show the amount of RAM each model can use.