No info shows how to even find memory modules on imac retina 21.5 in 2017 computer

You don't.

If you reread the article link above your name, you will see that, starting in 2012, memory modules in 21.5-inch iMacs are not user-accessible. The case is sealed and the RAM slots are on the "back" side of the logic board, requiring nearly a complete gutting on the computer to access. Apple won't do it, and even some Apple Authorized Service Providers will not tackle the difficult work. Attempts to break the seal and disassemble opens a Pandora's box of possible severe and even fatal damage to the computer.

If you want more RAM because the computer is "slow," you are probably pursuing the wrong suspect. Based on a long sad history of sealed-case 21.5" iMacs here, about 95 percent of the time. "slow" is not due to RAM starvation but rather due to a slow and under-spec mechanical hard drive used from 2012 through 2019. You can throw massive of RAM at the issue and not change a thing.

So do a quick Q&A about your computer:

• Is it slow to boot?
• Are many apps very slow to launch?
• Once launched, do the apps seem to work normally?

Three "yes" answers screams "typical iMac mech HD problem." In that case there is an effective solution that costs less than having a pro add RAM, will actually improve performance, and does not require disassembly to implement.

Let us know how many "yes" answers you have.

A

jinarajja wrote:

yes yes yes

To confirm, I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as shown here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Jack

yes yes yes