For each question I'll answer in terms of lesson 7 instead of 6.

1) For vehicle references you can take a screenshot of a side / front view and draw over it in any digital drawing software for your orthographic plan. Here is an example of how you would do that https://drive.google.com/drive/folders/1NeE_AiE7szr_vJEicrHbQBe43dcu96kr (focus on ref study 1 and 2). You can also just eyeball how many wheels wide / tall it is to train your observation skills.

2) You decide proportions through your own observation and judgment and by drawing orthographic plans of vehicles based on their wheel lengths. Once you've drawn the plane of the orthographic using the wheel lengths you can just use subdivision + your observation + judgment to place everything.

3) Through observing the object and making your own decisions about how much space it should take up. Uncomfortable came to the conclusion that it was less than 1/3 but more than 1/4 through just looking at it. You could also have a different judgment about how much space it should take up and that would be equally valid because it's based on your observation.

4) This is again based on your own judgment and how precise you want to be.

5) Orthographic plans are only drawn in 2d. The final vehicle should be drawn in 3d and based on the orthographic plans.

6) No, that is just observational drawing (drawing what you see) drawabox uses a construction method which involves observing a subject, understanding how it sits in 3d space, and drawing your understanding of the object in 3d space.