Views has base template views-view-unformatted.html.tpl with contents:
{% if title %} <h3>{{ title }}</h3> {% endif %} {% for row in rows %} {% set row_classes = [ default_row_class ? 'views-row', ] %} <div{{ row.attributes.addClass(row_classes) }}> {{- row.content -}} </div> {% endfor %}
Where variable {{ title }} – mean The title of this group of rows. It’s usually empty.
So, I want to preprocess this template in my .theme file and override this {{ title }} variable (or assign new) with my contents for this view if grouping enabled.