The Products in Category section lists the products that are currently assigned to the category. As a Magento theme, ScandiPWA supports the ability to assign products to a category through category settings.
Feel free to follow your regular steps and ScandiPWA will respect all the changes related to products in a category.
The products will be updated on the Front-End immediately after save as they are part of the ScandiPWA caching solution that will invalidate caching entries for the updated content. No cache flush or any other manual operations are necessary. Content will be visible after page refresh and new cache entry will be stored.