Hello! I am glad you liked our game :)
The background is a regular image that is repeated on the X axis, its offset matches the player's rotation angle. Then, 3D cubes and their bounding boxes are drawn using raylib's 3D mode (which uses OpenGL), the trick here is to use a very small render texture (of the same size that the Nokia screen), because drawing 1-pixel lines always works well; only solid colors, no lighting effects. The 3D is drawn with a 3D camera that's placed exactly behind the player, and looking in the same direction. Afterwards, the player itself is a single sprite on top of the image.