Taken from our friends at '''EveryMac.com''':
[quote]Apple reports that this model has memory "built into the computer" and "it can only be upgraded by an Apple Authorized Service Provider if you choose to upgrade at a later date." It is possible, but difficult, to upgrade the RAM yourself, too.[/quote]
They have a detailed write-up on how to upgrade your iMac’s RAM here: https://everymac.com/systems/apple/imac/imac-aluminum-tapered-edge-faq/how-to-upgrade-imac-ram-aluminum-2012.html
Here are the specs for this particular model, where it’s stated the max allowed RAM is 32 GB: https://everymac.com/systems/apple/imac/specs/imac-core-i3-3.6-21-inch-aluminum-retina-4k-early-2019-specs.html
I hope this helps!