Hi, I see that you and David are doing nicely!
Just to clear up one thing: Position relative means you can position something relative to where it would have ended up if you didn't use the positioning. Move it up, down, left or right relatively to it's normal positioning, e.g.
And to your questions above:
The black body background doesn't display more than 200px height because there is no content in the rest of the body. (well, there is, but you have commented it out, so it doesn't display.)
There is no conflict between the body width and the container width. But giving the body a width is not necessary, because look at it as the background of the whole site. If someone has a small monitor or a large one, the body will spread out wall to wall anyway. The container (someone also call it 'wrapper') is where you place the site content, and it should have a width that doesn't cause anybody having to scroll sideways to see all the content. The norm these days seems to be something like 950 - 1000px.
You can create 'containers' inside THE container too, but then you have to name then differently, like e.g. 'main' and 'aside' if you want a two-column page.