Here are the latest estimated views showing the internal layout as discerned from video and other evidence:
There have been many iterations of these sketches and the 3D model, but each has differed in minute detail that only the most enthusiastic observer might notice.
The original schematic/cutaway sketch was by the blogger Plane Man. We can now see that it was wrong in almost every detail, but correct in many ways too. It acted as the starting point of this project.
Although the original source is not clear (if you know please tell us so that we can give credit!), this South Korean sketch which we found during research turns out to be very good, although not exactly as per the IS-120 model: