Right, they get archived just like you described. Once the thread is 7 days old it automatically gets moved to the other forum, mainly so that the original forum doesn't get completely clogged with 365 of these threads after a year has gone by.
The threads continue to be available as always, and can continue to receive replies. Members who participated in the threads can also continue to view them via their thread watcher, "My Posts" feature, and of course just going to that archive forum.
