So it's worth drawing a distinction between how we approach conveying the forms that exist inside of the existing silhouette of your main form (the sausage), and how we approach conveying forms where they break beyond it. For the former group, we use cast shadows (which I will note the demonstrations do not convey very well, because this focus on implicit markmaking using cast shadows is something we pushed towards gradually, and which the demonstrations won't fully convey until our overhaul of the demo material reaches that stage - right now we've just completed Lessons 0 and 1).

For the latter, you would indeed draw the outline of the new silhouette where the form breaks past that surface, as noted in this section of the notes. This would be conveyed as outline, rather than your suggestion of casting other shadows outside of the form, as every shadow requires a surface upon which to be cast, and there would be no surfaces outside of the sausage to receive it in this exercise.

As to your second question, a lot of it comes down to context. The classic pattern of staggered rectangles spaced out with an even border that people tend to picture would not constitute a texture, but the physical bricks themselves, arranged along the plain flat surface of a wall would indeed be a texture. Looking more closely, the bricks themselves would also have texture resulting from the unevenness of their surface, the little pockmarks, scratches, erosion, etc.

In this section I explain it as follows:

Texture - that is, what people tend to think of as detail - isn't actually all that different. While we treat it a little differently, texture is also made up of three dimensional forms. The only difference is that these forms adhere to the surface of some other object - and this difference fundamentally changes how we approach drawing it.

An example I like to use is fish. If you've got a fish swimming in the ocean, then we draw it similarly to how we draw the boxes and sausage forms we've tackled thus far. We apply constructional means - drawing through our forms, defining their silhouettes with outlines, describing how their surfaces move through space with contour lines, etc.

If, however, you take a bunch of fish and use it to wallpaper your bedroom, it becomes a texture - and the way we draw it changes. The fish is now a part of the wall itself. If the wall turns, the fish will follow. If you were to strip down this fishy wallpaper and wrap it around a box instead, the fish would come along with it. They cease to be an independent object, but rather become a part of this texture that can be applied to any other surface.