Thank you for the kind words! Jumping right in, your form intersections are looking quite solid, and demonstrate a good grasp of the relationships between the different forms in 3D space. Similarly your cylinders in boxes are generally coming along well (the choice not to use an ellipse guide here was a good one, as it very much demands that we have our ellipses touching all four edges of their enclosing plane, as the ellipse is a tool for testing the proportions of that plane) - but I did notice that you appear not to be applying the line extensions for the minor axes here. Rather, you appear to only be extending the contact point lines for each ellipse, leaving one spot for your errors to go unnoticed. Make sure you're applying the line extensions completely (3 lines for each ellipse, all extended all the way back) so you can properly test your boxes' proportions.

Moving onto your vehicle constructions, I can certainly see that you've knocked it out of the park in terms of working carefully and gradually, applying plenty of subdivision, and really holding to the principle of ensuring that you're giving yourself as much time to apply your understanding of the concepts to the best of your ability. That matters a lot, in the context of giving critique, as it allows me to zero in on the specific areas you may be struggling with, rather than trying to judge whether something was understood but rushed.

The first issue that jumped out at me was that the proportions for this one were definitely looking wrong, which suggests that there's some step in the process of setting up that grid that was not followed correctly. I went in over your work in photoshop to try and find the minor axis alignment for the ellipses you used when setting up the bounding box (I have an ellipse guide tool that allows me to identify the properties of the ellipse with greater specificity), and I found that as shown here the ellipse on the left side (the one helping to establish the width of the front face of the bounding box) was off.

With the minor axis properly aligned towards the right vanishing point, we get a narrower ellipse (once the degree is adjusted to account for the contact point alignment). While this isn't much narrower, once we apply the extension of that measurement back in space, it compounds and results in almost a full span being cut off that side of the box.

The short of it is to be a bit more attentive to those ellipses - they can be tricky, and it can throw things off a great deal.

Now I admittedly said "the first issue..." because I kind of expected there to be at least one more, but I hadn't scrolled further down yet. Unfortunately I'm left a little short here - your work demonstrates a level of patience and care that I don't really see that often. I know I mentioned this earlier, but I had not at that point seen examples like your space shuttle, or your double decker bus. As much as this is exactly what I asked for, it's not often that I see it applied to such a degree.

Fortunately, I do have one last bit of advice to offer - when going in to add cast shadows and other such embellishments, it's important to ensure that the tools you're using match - in terms of colour, and even in terms of weight. A black ballpoint pen with fineliner used to fill in sections can work, as long as ballpoint is used to add enough line weight to "bridge" the difference (rather than jumping from ballpoint's naturally thinner marks straight to a much darker shape). Jumping from blue to black though is inevitably going to look kind of discordant. In that case I might just stick with the blue pen, and use it to fill in my shadow shapes.

Additionally, remember that those filled areas of solid black should be reserved for cast shadows only - if you find yourself trying to fill in an existing shape in your drawing (like here where it seems like you're filling in the side planes of the structures between the bus windows), then that is more in the vein of form shading where the orientation of a given surface dictates whether it's lighter or darker. Cast shadows require us (at least in most cases) to design a new shape, one that defines the relationship between the form casting the shadow and the surface receiving it.

Lastly - and this is more just a matter of taste - but I find that while filling in cast shadows on the construction itself works well enough (if the previous points I'd mentioned are met), often times the cast shadow on the ground ends up being a bit distracting. You might find it comes out better if that shadow on the ground is merely outlined, rather than filled in.

And that covers it! All in all you've done an excellent job, and I'm very pleased with your results. I'll go ahead and mark this lesson - and the course with it - as complete. Congratulations!