To be fair, as you say, the only upgrade you can make to an Android phone (other than the LG G5) is to stick a bigger SD card in it (or battery in a few older models). And not even that in some cases.
Put simply, all of the hardware is soldered or bump-bonded to the motherboard. Even if you could find newer replacements that were pin-compatible, good luck replacing them and still having a working device afterwards (somebody apparently did manage to upgrade the storage of a Nexus 5X this way, but that's simpler than replacing the SoC or RAM, and still requires a lot of skill). And then there's the problem that the system software was built for the hardware the device came with, so if you change that there is a good change that your device wouldn't even run iOS afterwards.