The whole source code is in there, H & CPP.
Any struct defined as USTRUCT() and marked SaveGame when declared as UPROPERTY() is supported; Inclusive Structs created as Blueprint Assets from Browser.
If you want to save raw CPP types, it’s a good idea to study the “Reflector.h” and “Reflector.cpp” files.
You can add custom serialization functions to it to save any kind of data you need.