I need to upgrade storage on 5K, 27in 2020 Imac - what should I purchase.

ok. well upgrading the internal drive is not recommended. it requires partially disassembling the Mac, including the screen, which is held on with adhesive tape.

if you are in need of extra storage, you could get an external HDD or SSD of sufficient capacity and store your data there. and once the files are on the external drive, you can simply erase the same files from your internal drive. and, if it's just going to be used for storage, you can use an HDD and save a few bucks over the cost of an SSD. FWIW, i personally keep very little on my internal drive. additionally, you want to keep ~ 20% of your local drive as free space for proper operation of macOS.

but please note that you do not want to use any preloaded software with the drive that supposedly makes the drive "Mac ready". when you use software from just about any manufacturer for their storage device you run the risk of that same software not being compatible with macOS updates / upgrades. it can cause the software to stop working. at best this will cause you a headache. at worst it can render your data inaccessible. it is always best to use disk utility to erase and format a drive before use to remove any preloaded software. please see Erase and reformat a storage device in Disk Utility on Mac - Apple Support

your title says storage, and your question is asking about RAM. which is it that you are asking about? please clarify.

in either case, I would shop at OWC’s website. their main focus is Mac compatible hardware.

This is confusing. The title of your question says 'storage' which would mean disk storage, as in a hard drive or SSD. But your details are about RAM DIMMs, which is 'memory,' it is not 'storage.'

If you want to add RAM memory, it depends on the exact model iMac you have. Do Apple > About This Mac and tell us what it says for Model & Processor. OWC and Crucial are good vendors.

If you want to add disk storage, you should consider a good external drive that you would connect to one of your iMac's Thunderbolt or USB ports. OWC has many good drives (here is one example.)