For example:
Say i have a game that i plan o releasing on steam and EGS.
Do i need to build the game twice (once with the Steam Online subsystem and another time with the EOS)? Or can i build it once with both? If i can build it with both how can i determine wich subsystem to use?
Presumably you want to talk to the Steam system? Do you also want to talk to Epic?
EDIT: Sorry there’s not enough detail to answer this easily.
If it’s just windows ( for instance ), it’s conceivable you could write one version which could figure out which platform it was purchased from and go from there. Otherwise, you’ll need to put the platform specific code into a plugin which you can load as needed.
Steam gives you a way of packaging the game so all the main parts are unchanged ( PAK etc ), and you can then install the other parts you need from a cache ( correct EXE, plugins ).
( I don’t know how the Epic store packages and installs things. )