You can see examples of the first Platform API shown by Casey in SDL and cocos2dx code.
SDL has a function called SDL_CreateWindow that internally calls the platform dependant functions to create the window: https://hg.libsdl.org/SDL/file/6c469ea796e4/src/video
You can see that for every different platform there's dedicated folder that do the actual code (SDL_video.c has that function)
In cocos2dx, there's the FileUtils class that the actual implementation is picked at compile time. https://github.com/cocos2d/cocos2d-x/tree/v3/cocos/platform
Actually cocos2dx uses both techniques, they have a platform layer that calls the init part of your game but then you have to call other parts of the platform api yourself.