To set your expectations, this request falls under custom work. However, you can try the following solution.
Please copy the file buddyboss-theme/buddypress/common/nav/directory-nav.php into your child theme folder, including all subfolders. Once copied, replace the <ul> tag (from line 14 to 33) with a <select> tag to convert it into a dropdown.
Here’s the code snippet to use:
<select class="component-navigation <?php bp_nouveau_directory_list_class(); ?>" onchange="location = this.value;"> <?php while ( bp_nouveau_nav_items() ) : bp_nouveau_nav_item(); ?> <option id="<?php bp_nouveau_nav_id(); ?>" class="<?php bp_nouveau_nav_classes(); ?>" <?php bp_nouveau_nav_scope(); ?> data-bp-object="<?php bp_nouveau_directory_nav_object(); ?>" value="<?php bp_nouveau_nav_link(); ?>"> <?php bp_nouveau_nav_link_text(); ?> <?php if ( bp_nouveau_nav_has_count() ) : ?> (<?php bp_nouveau_nav_count(); ?>) <?php endif; ?> </option> <?php endwhile; ?> </select><!-- .component-navigation -->