Confusion about the Beginner's WP Course

I’m enrolled in the Beginner’s WP course and am in the section on Widgets and Menus. I’ve been doing great up until the point where the instructor says

“Here are the modifications I’ve made and the new additions and modifications in code form:

I modified #site-content to decrease the max-width to 525px and then added left float to make it float left.

I added #site-sidebar giving it a 220px width, 22px border, a border radius and white background just like the content and I floated it to the right.

I added #site-container making it as wide as the cumulative width of the content and the sidebar and centered it.

I added a clear rule to the footer to force it below the floated elements.”

Here are my specific issues:

1) Up to this point, there have been complete source code files for me to look at, but not here, it seems. Are they available?

2) I’m not sure what he means when he says “added a clear rule to the footer”. Source code examples would clarify this.

3) How did he center the #side-container?

4) Is there a discussion forum specifically for these courses? There’s a discussion at the end of each section, but you have to be able to complete the section to get to it. How do I ask questions while working through a section?