It would help if you gave your exact model number.My iMac 2015 has 1-terabyte fusion drive containing a small 24-gig SSD. Can I upgrade the internal SSD to a much larger SSD, making the internal hard drive for storage only as a separate partition?
I'm guessing it is a 21.5" Late 2015 iMac16,1 or iMac 16,2. On that model the SSD is a PCIe "blade" card.
If you can find a compatible replacement (which isn't easy; the product lookups are assuming you want to replace the hard disk drive), you can swap it. But the SSD card is on the back of the logic board. The replacement process is... daunting.