How to install additional memory in my iMac

Adding RAM to any 21.5-inch iMac made after 2011 is impractical.

Apple did not design the case to be opened by users. The RAM slots are are located such that, even if you get the case open, you must gut the computer to access the slots. The chances of collateral damage in the process are very high based on numerous sad posts here.

Are you running ant anti-virus or so-called "cleaning" apps? Some can cause spurious alerts.

Before you draw the conclusion your Mac needs more memory, confirm: Check if your Mac needs more RAM in Activity Monitor - Apple Support. If its "Memory Pressure" graph is frequently "red" your Mac would benefit from additional memory. If not, it won't. It's that simple.

Report your observations. There are reasons for the "Your system has run out of application memory" dialog for which memory is only indirectly related.

The RAM in newer 21.5" iMacs is non user-accessible. They have to be completely disassembled to add memory. Preferably by an Apple Authorized Service Provider who is willing to do it. They charge accordingly.

If you don't have enough free space on your hard drive fore memory cache files you'll get the application out of memory waring. How much free space do you have on your boot drive?

What model and year is your iMac?

FWIW, you can join Zoom meetings in a browser but I don't think the app is the issue. You haven't answered OT's question about how much available space is on your drive.

Here's the info on joining a Zoom Meeting in a browser.

How much free space do You have on your boot drive?

You can get an external drive and move files to it so as to free up space on your boot drive.

The files that you have control over are located in the Documents, Downloads, Pictures, Music and Movies folders.  You can use this free app, GrandPerspective, to find the largest files on your drive so you can determine if they can be deleted or moved to an external HD for storage.

Note: you can empty the Downloads folder after the apps and/or updates that were downloaded have been installed or applied.  Some users have found a couple of Gigabytes of files in their Downloads folder which are no longer needed.