Shoutbox

Depends on the creator of the site. Usually, professional creators have some method of replicating the same content, as far as my limited knowledge goes.

I did once use javascript (I think) to make a "template", and now I use PHP includes. Saves me the hassle of updating every page.

If there isn't a root code, then yes, you do have to manually change it. Or, if the code on every page is the same, then just use find and replace. (If you're modifying, you could might as well make a "root".)