2019 21.5 iMac memory upgrade
will they do it at an Apple Store?
iMac 21.5", macOS 10.15
No! You need to contact an independent AASP and ask if they will upgrade the RAM. This will be expensive as it requires a complete disassembly of the computer. If you have not purchased the computer yet, then just order it with 16 or 32 (recommended) GB of RAM.