Creating WordPress Plugin solely for Admin panel with dash menu and submenus

Here is a simple example to get you started with the right code. This creates a main menu item using add_menu_page then attaches a submenu using add_submenu_page. Both of them call a different function for the output.

Notice that the add_submenu_page function ties into the parent menu using customteam which is the $menu_slug of add_menu_page.

add_action( 'admin_menu', 'register_my_custom_menu_page' );
add_action( 'admin_menu', 'register_my_custom_submenu_page' );

function register_my_custom_menu_page(){
    add_menu_page( 'Team Kit', 'Team Kit', 'manage_options', 'customteam', 'my_custom_menu_page'); 
}

function register_my_custom_submenu_page() {
    add_submenu_page( 'customteam', 'Team info', 'Team info', 'manage_options', 'my-custom-submenu-page', 'my_custom_submenu_page' ); 
    add_submenu_page( 'customteam', 'Crew Stats', 'Crew Stats', 'manage_options', 'my-custom-submenu-page_2', 'my_custom_submenu_page_2' );
    //add_submenu_page_3 ... and so on
}

function my_custom_menu_page() {
    echo '<p>Hello, I am Team Kit</p>';
}

function my_custom_submenu_page() {
    echo '<p>Hello, I am Team Info</p>';
}

function my_custom_submenu_page_2() {
    echo '<p>Hello, I am Crew Stats</p>';
}