The address bar shows the URL that is going to be requested.
The address bar is usually populated once you click a link (or type something in).
htaccess cannot change the links on the page, it acts only on requests once they arrive at the server.
So, if something requests an "incorrect" URL, rules in htaccess can detect the request and respond by sending 301 status and the details of the new URL.
When that new URL is requested by the browser, the URL in the address bar is updated.