wp add menu page and subpage

wp add menu page:

function custom_menu_page() {
    add_menu_page(
        'Custom Menu Page Title',
        'Custom Menu',
        'manage_options',
        'custom-menu-slug',
        'custom_menu_page_content',
        'dashicons-admin-page',
        6
    );
}
add_action('admin_menu', 'custom_menu_page');

function custom_menu_page_content() {
    echo '<div class="wrap">
        <h2>Custom Menu Page Title</h2>
        <p>Custom Menu Page Content</p>
    </div>';
}

wp add menu subpage:

function custom_menu_subpage() {
    add_submenu_page(
        'custom-menu-slug',
        'Custom Subpage Title',
        'Custom Subpage',
        'manage_options',
        'custom-subpage-slug',
        'custom_subpage_content'
    );
}
add_action('admin_menu', 'custom_menu_subpage');

function custom_subpage_content() {
    echo '<div class="wrap">
        <h2>Custom Subpage Title</h2>
        <p>Custom Subpage Content</p>
    </div>';
}