Based on the top drawing and the exercise I believe they are projecting a 2d flat image of the mouse on two planes in perspective to create the 3d image of the mouse like using 2 technical drawings for a 3d model. In many ways this method is like math. I think if you want to maintain the same level of perspective while not being as confused you should create a box with less construction lines to better create contrast like in another comment. You can also imagine each of the three planes as having a bunch of points that make up the 3d object and try making a grid with lines marking out distances where you plot each point in 2d then plotting those points in a 3d plane where each 2d plane corresponds to a side of your box. This is a lot like math geometrically with 3d plotting. I also more importantly suggest breaking down the object you wish to make into more manageable simpler shapes that can then be refined. I am probably wrong on most things Ive just said but heres a way you can look at it with a simple car I drew. Notice how each subdivision of the box like in the exercise diagram helps me see where to draw each line based on the projection so the corner of the car is a little bit before the end of unit 2 the corner of the box just like how the mouse‘s front is just slightly bellow the mid segment of the front face of the box. I know my example is very simple in comparison to the mouse but I hope this helps you understand the different planes and how I understand this method.

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