Add subcategories to posts category page in header

Modify the template used for category archives by adding wp_list_categories(). Pass either the “parent” (immediate sub-cats) or “child_of” (includes sub-sub-cats, etc.) argument using the current category’s ID as the passed value.
https://developer.www.ads-software.com/reference/functions/wp_list_categories/

The arguments you can use are fully defined at
https://developer.www.ads-software.com/reference/classes/wp_term_query/__construct/#parameters