OnlineSubsystemSteam - where are those .cpp and .h files?

Hey, in the documents it says for all steam functions:
Module: OnlineSubsystemSteam
Header: Runtime/Online/OnlineSubsystemSteam/Public/OnlineSubsystemSteam.h
In my sources, I don’t even have those OnlineSubsystemSteam folders and even not any .h files. Where can I find those or where can I download these? I have already implemented the SteamAPI and it’s showing up the Steam overlay too, but I can’t access any Steam functions. Please help!