This is doing the same thing as the V8AndGUI demo, except that no scripting engine is used, and everything is in C++.
The "scripting" code are compiled into DLLs, which can be dynamically loaded at runtime.
APIs are wrapped and exported first using extern "C", then wrapped again using a set of header-only classes, which are derived from std::shared_ptr.
This is just like a typical plugin pattern, so nothing much to say.