Will the wonders ever cease? Apple initially includes NFC, now new iMacs have processor and RAM that can be added. The 27-inch iMac version from 2015 made it easy to upgrade RAM, but 21.5 "was shut down. This varies with the new 21.5 "models, even if it is not an easy process.
The 21.5 "iMac comes with a dual-core i5, which will get old soon. Of course, Apple offers limited space support, so you can not capture any CPUs compatible with 1151.
However, the AMD Radeon Pro GPU is attached to the motherboard. However, Mac OS High Sierra supports external GPUs on Thunderbolt 3, so you should be good at graphics (however it will cost you).
As for memory, 2.5 HDD is easy to upgrade, but any flash space is adjacent.
