Re: how the bbpress’s front page controller works?
Actually MultiViews looks for topic.php if it hasn’t found topic/, but otherwise you’re spot on.
You can also use RewriteRules which forces the server to do the same thing ‘manually’.
When you request that URL, Apache’s MultiViews or RewriteRules (whichever you are using) loads topic.php. That file then prepares a bunch of data and displays it with my-templates/topic.php (If that file exists) or bb-templates/topic.php (otherwise).
If you are familiar with the way WordPress works, it may not help much This is one area where WordPress and bbPress are quite different.