Not user-replaceable in Apple-speak means "you have to take the back (or the front) off to get to it". iMacs come in three difficulty levels to get at the RAM - apart from a few which use soldered RAM, and are literally impossible to upgrade short of changing the motherboard (only viable if you have a higher-RAM iMac of the same model with, say, a cracked screen).
Just for context, I am NOT a technician, but I'd consider myself an advanced user, and have been inside a lot of computers and even built a few over the years. I'd comfortably change memory or a drive in most computers where it isn't soldered in, and I'd change a motherboard on most desktops and even many laptops.
We know the iMac Pro isn't in the easiest group - the "Apple provides a hatch, and considers the RAM user replaceable" cohort that includes all 27" models except the iMac Pro, and certain others (many of the smaller aluminum models before the tapered edge generation). These are easy - somewhere between inserting a video game cartridge and changing a video card in a tower PC. Very little of the computer is exposed - just the RAM slots. Actually getting the DIMMs to seat ranges from easy to frustrating depending on the model. I can get most of the newer ones first time, every time - but some of the early aluminum models with the hatch on the bottom instead of the back had tricky slots where it was easy to get the RAM halfway in and have to open up again to reseat it.
Some of the supposedly non user-replaceable RAM in iMacs is, in fact, not that hard to do.The intermediate difficulty group involve taking the back of the Mac off. I've done a bunch of these, and can do them in about half an hour with a negligible chance of breaking the machine (I'd do one that was still under warranty, no problem). The warranty (of course) doesn't cover you if you break it while replacing the RAM, but you don't lose the warranty if you do the RAM successfully and something else later breaks. You do fully expose the motherboard, just like replacing RAM in a regular desktop, and you sometimes need to be careful of one or more stupid little cables leading to the rear I/O ports. The RAM is on the first side of the motherboard you see, so you don't have to take the motherboard out.
Other iMac models (including the latest 21.5" models) can only be gotten open by removing the screen, and that's much scarier. I've never done one of these, and I'd only be willing to do it on a machine that was no longer under warranty. Basically, you pull the screen off with suction cups (after cutting some tape), then end up facing the wrong side of the motherboard. You have to remove the motherboard to get at the RAM slots, then reassemble everything. That's a huge pain, and it's possible to crack the screen in the process.
If you happen to have a motherboard with more RAM handy, the impossible iMacs are actually slightly easier than the difficult ones - they're also accessed through the screen, and you still take the motherboard out. The only difference is that you simply exchange the motherboard instead of adding RAM before putting it back in. The only way it would be economical is if you had a compatible motherboard with more RAM from an iMac with a different problem (or possibly if you could somehow get ahold of one as a service part that had both more RAM and a substantially faster processor - there may be a model where you could go from dual-core to quad-core - it looks like this may be true in some of the Late 2015 models?) It's unlikely that you'd ever exchange a functioning motherboard just to add RAM.
We know the iMac Pro is not in the impossible category - the RAM is, at least, socketed. The access to all other tapered edge iMacs is through the screen; and I don't see any screw holes on the back, so I'd guess this one may be as well. Who knows what else is socketed? It looks like, under the cooler, the processor might be. I'm assuming the two plates leading off the cooler in the middle are the CPU on the left (between the RAM sockets) and GPU on the right. There have actually been a few iMacs (and numerous other Macs) where socketed processors made upgrades possible. It would make sense if the processor were socketed - not least because it saves the expensive (as much as $2500 in the case of an 18-core) Xeon when a 35-cent USB port blows and needs a motherboard swap. If the SSD is socketed, it is either under the daughtercard by the GPU or on the screen side of the motherboard - it's not visible in the standard internal image. The GPU is almost certainly highly non-standard - it may be socketed, but good luck finding anything that fits!
I wouldn't try upgrading one of these myself unless the back turns out to come off - if you do get in through the screen, the RAM is on the wrong side of the motherboard - it clearly faces the back of the machine. A $5000 - $13000 machine that requires pulling a 27" screen off with suction cups and removing a motherboard that seems to have hoses connecting the CPU and GPU to a cooling system (is it liquid cooled?) seems to be the kind of nightmare best left to Authorized Service Providers.