First is this the site and system you where looking at: Refurbished 21.5-inch iMac 3.1GHz Quad-core Intel Core i5 with Retina 4K display
So lets dig into what they are saying. Here's the exact system: iMac 21.5" 3.1 GHz i5 Retina 4K (Late 2015)
So the original release date of the system was October 2015. But it was sold until June 2017 when the new models arrived.
Sadly, this model only offers 8GB memory of soldered down memory so it's not upgradable. In addition this model does not offer the needed PCIe slot hardware on the logic board to add in a second drive (blade SSD).
Here's the IFIXIT teardown for this system which you can review: iMac Intel 21.5" Retina 4K Display Teardown
As to upgrading any of the 21.5" 'Thin Series' iMac's getting to the RAM would require completely removing the logic board as you can see here in this IFIXIT teardown of the older 21.5" iMac: iMac Intel 21.5" EMC 2638 Teardown. At this time point Apple still had socketed RAM and you'll also see the PCIe blade SSD socket in this image: PCIe SSD socket.
Now lets jump to the newest 21.5" IFIXIT teardown: iMac Intel 21.5" Retina 4K Display 2017 Teardown. Here we can see Apple gave us the RAM slots back! But if you get the HD only model you don't get the PCIe slot to add the blade SSD later.
But in any case its still a bear to upgrade the RAM as you would still need to take the logic board out to gain access to the RAM slots. I should also point out getting the display off with the newer 'Thin Series' is not easy and if you mess up you can damage the display. So you do need to use the correct tools and follow the IFIXIT guides to the letter.
You also appear to be looking for a system which supports a lot of RAM (32 GB). Often when I see someone looking for this much they are doing image processing or video editing. In either case I would recommend going with a 27" model so you have a larger display to work off of and unlike the 21.5" model has a hatch to access the RAM slots! As you can see here: iMac Intel 27" Retina 5K Display RAM Replacement and besides with the newer series you can boost the RAM to max of 64 GB!
As to storage I would recommend you get the largest PCIe Blade SSD model you can afford then use an external Thunderbolt RAID drive to hold your work.