I have a late 2013 iMac with 16GB RAM and upgraded 2.9 i5 processor that had a Fusion Drive setup.
I added a new 2TB SSD last month. I also added a 500GB blade SSD from a MacBook Pro that I had spare from my late 2013 MacBook Pro as my iMac version has a blade SSD slot on the logic board.
To get to the RAM and the Blade drive I literally had to take the whole Mac apart and remove the logic board as both the RAM slots and the Blade type drive are not accessible without doing this. I used ifixit’s kit (not a plug) and it was a long job but can be done as long as you are careful when removing the display assembly and also when putting it back on.
If you only want to upgrade the normal SATA HDD to a SSD it doesn’t take as long as you only have to take off the Display assembly.
I have recently added a photo of the logic board I took out from my iMac below when I was taking it apart for anyone wanting to see what it looks like after removing. You can see the slots for the RAM and the heatsink for both the processor and GPU as well as the PCIE slot at the top of the picture. This was after I had just taken it out so you can see the dust at the top on the USB connections.