Start with default generated template

templates/views/views-view-summary.html.twig

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+{#
+/**
+ * @file
+ * Theme override to display a list of summary lines.
+ *
+ * Available variables:
+ * - rows: The rows contained in this view.
+ *   Each row contains:
+ *   - url: The summary link URL.
+ *   - link: The summary link text.
+ *   - count: The number of items under this grouping.
+ *   - attributes: HTML attributes to apply to each row.
+ *   - active: A flag indicating whether the row is active.
+ * - options: Flags indicating how the summary should be displayed.
+ *   This contains:
+ *   - count: A flag indicating whether the count should be displayed.
+ *
+ * @see template_preprocess_views_view_summary()
+ */
+#}
+<div class="item-list">
+  <ul class="views-summary">
+  {% for row in rows %}
+    <li><a href="{{ row.url }}"{{ row.attributes.addClass(row.active ? 'is-active')|without('href') }}>{{ row.link }}</a>
+      {% if options.count %}
+        ({{ row.count }})
+      {% endif %}
+    </li>
+  {% endfor %}
+  </ul>
+</div>