Login/Logout Redirect

You will need to hook into the login_redirect filter.

>But wasn't the redirection placed in wp-login.php?

— yes, but you shouldn’t edit core WordPress files, especially since there is a hook that you can use instead.

Your code should look something like:

add_filter('login_redirect', 'my_login_redirect_function');
function my_login_redirect_function($redirec_to) {
    // add code here to determine where to redirect the the user after login

    return $redirect_to;
}