WordPress updates the post/page modified time only when you save/update the post/page in the editor.
If you have shortcodes or blocks that create dynamic content (ie. returning content from queries, files, feeds, etc.), the post/page modified time will not reflect these dynamic content changes.
JSM's Accurate Modified Time for SEO plugin monitors the post/page content for changes and updates the post/page modified time as required.
If you use a social and search optimization plugin like WPSSO Core, Yoast SEO, The SEO Framework, etc., the Open Graph og:updated_time and article:modified_time meta tags, along with the Schema markup dateModified property, will show an accurate modification time if your post/page content changes.
