I was nervous buying the the late 2015 iMac 27" because I could fine such little information about it. I could only find a few videos specific to the tear down and very little info about adding or upgrading to an SSD. There was tons of info about the 2014 and early 2015 model, but noting on the late 2015. So here we go.
Although difficult to open and with little space inside, you can still add a SSD inside the late 2015 iMac 27". In my current method you will be replacing the built-in drive, as we are using the built-in cables.