Start with default generated template

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

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+{#
+/**
+ * @file
+ * Theme override to display a single views grouping.
+ *
+ * Available variables:
+ * - view: The view object.
+ * - grouping: The grouping instruction.
+ * - grouping_level: A number indicating the hierarchical level of the grouping.
+ * - title: The group heading.
+ * - content: The content to be grouped.
+ * - rows: The rows returned from the view.
+ *
+ * @see template_preprocess_views_view_grouping()
+ */
+#}
+<div class="view-grouping">
+  <div class="view-grouping-header">{{ title }}</div>
+  <div class="view-grouping-content">{{ content }}</div>
+</div>