iMac late 2015 memory is not upgradable?? What?

It's soldered to the motherboard on the 21.5-inch models.

(219918)

Do you want to add RAM because your computer is slow? Forget it. Adding RAM to any "slow" 21.5-inch iMac made between 2012 and 2019 fails to fix "slow in about 99% of the cases, making this largely a moot point.

What makes those iMacs slow is the lap-top class 2.5-inch, 5400rpm., SATA 3GB hard drives Apple used for the base level storage. They are no longer fast enough. Newer macOS versions are struggling with old mech drives. That statement is based on thousand of reports here.

There is a simple three-question test that can help tell if the hard drive is your bottleneck:

• • Is your Mac slow to boot up?
  • Are many apps, even System Preferences, slow to launch?
  • Once launched, do the apps seem to run normally?

Three "yes" answers scream "under-spec factory hard drive" as the source of the "slows." There is a cost-effective DIY option that that may be appropriate. But the decision to go there depends on date we don't have at this point.

If you want a data-driven analysis of you computer's overall performance status to see if the DIY option is appropriate to your problem, please post a configuration snapshot. Fortunately there is a safe, secure way to do that. 

We can quickly and within the confines of these forums help you determine the issues at play if you use EtreCheck Pro, available here:

https://etrecheck.com/index

The free version will do nicely for this purpose, although the app is worthy of our financial support.

We can see hard data about drive performance, software issues, and RAM usage. Etrecheck is the development of a long-serving and trusted contributor here expressly for displaying information in these forums to help us help you. It is the single best tool for evaluating computers that, in this text-based venue, we can neither see nor access remotely. It will not reveal any personal or secure information.

See this excellent user tip on posting text reports like EtreCheck:

How to use the Add Text Feature When Post… - Apple Community

That is correct after the purchase. Upgrades had to have been made at time of option selection during the purchasing process.