Where to edit the template that is generating the code for dynamic_sidebar left-sidebar in the Understrap theme? [closed]

Dynamic_sidebar pulls whatever widgets are assigned to that sidebar. If you want to add a class for the whole sidebar, surround the call in a div that you can then assign classes.

<?php if ( is_active_sidebar( 'left-sidebar' ) ) : ?>
    <div class="sideclass">
        <?php dynamic_sidebar( 'left-sidebar' ); ?>
    </div>
<?php endif; ?>