How do I demote a menu item in the admin menu?

Your question is very abstract. It actually vary form plugin to plugin how the organize the menu and pages. So it’s kinda hard to know how the plugin developer has managed the pages in their code. By the way as far I understood, you need to remove the page by using remove_menu_page function. Here is an example of it-

function custom_menu_page_removing() {
    remove_menu_page( $menu_slug );
}
add_action( 'admin_menu', 'custom_menu_page_removing' );

Here you’ll get a documentation. Please follow that.

Then you need to add the page to settings by using add_options_page. Here is

add_action( 'admin_menu', 'my_plugin_menu' );

function my_plugin_menu() {
    add_options_page( 
        'My Options',
        'My Plugin',
        'manage_options',
        'my-plugin.php',
        'my_plugin_page'
    );
}

And for adding to Tools menu use add_submenu_page.

The documentation is here.