You can quickly add a sitemap to a page or post by using the sitemap shortcode, in any theme. Just add this shortcode to any page or post using a shortcode block:
The shortcode will display an outline of all your site’s pages in a hierarchical bulleted list, following any parent/child relationships set in Page Attributes. It will look something like this, but with your own pages, and the styling will depend on your theme:
You can also use the sitemap shortcode in a text widget, although the display may not be bulleted or indented, depending on how your theme is designed.