What wordpress theme page alters the display of category pages

I wanted to show subcategories on my categories page with this code:

if (is_category()) {
  $this_category = get_category($cat);
  if (get_category_children($this_category->cat_ID) != "") {
    echo "<h1>Subcategories</h1>";
    echo "<ul>";
    wp_list_categories('orderby=id&show_count=0&title_li=
&use_desc_for_title=1&child_of='.$this_category->cat_ID);
    echo "</ul>";
  }
}

HTML:

but I don't know which theme file to paste it in