How to display posts from a specific tag in the current category on a single page

You need to first store your sub category id and pass that to WP_Query.
Below is the logic how we can store the child category value.

$categories = get_the_category();

// store the subcategory id
$sub_category_id = [];
foreach($categories as $category) {
    if( $category->parent > 0 ) {
        $sub_category_id[] = $category->term_id;
    }
}

$args = array(
    'post_type' => 'post',
    'category' => $sub_category_id,
    'tag' => 'city', 
    'posts_per_page' => -1
);
$query = new WP_Query($args);