Create pages in plugin

Hi everybody
I have a question, and I searched a lot of information on Internet but I didn’t find anything so I ask you for your help.
(If I have some grammar mistake, apology me cause I’m not English)
So, this is my first time to create a wordpress plugin.
I have the menu, my code is:

<?php
/**
* Plugin Name: Disponibilidad Habitaciones
* Description: Permite gestionar la disponibilidad de las habitaciones de los hoteles
* Version: 1.0.0
* Author: Gemma Pinyol
* License: GPL2
*/

include_once (‘funciones.php’);

function disponibilidad_habitaciones(){
//esc_html_e( ‘Disponibilidad Habitaciones Hotel Playa Golf Sitges’, ‘textdomain’ );
echo “<h3>Disponibilidad Habitaciones Hotel Playa Golf Sitges</h3>”;
seleccionarHotel();
}

function menu_plugin(){
add_menu_page(‘Disponibilidad Habitaciones’, ‘Disponibilidad Habitaciones’, ‘manage_options’, __FILE__, ‘disponibilidad_habitaciones’, plugins_url(‘/disponibilidad-habitaciones/hotel.png’,__DIR__),6);
//add_submenu_page(__FILE__, ‘Hotel’, ‘Hotel’, ‘manage_options’, __FILE__.’/habitacion’, ‘habitacion.php’);
}

add_action(‘admin_menu’,’menu_plugin’);

?>

Here I created the menu page.
My problema is that I can’t get in create a new page in the admin panel.
So for exemple I have a link to see the something that goes to a room.php page.
But where can I create the room.php to make it with the same structure that has the admin panel?
Cause if in the link is something like that:
Click here to see the room
If I click, the room.php page it is without the admin menú, the header, the footer, etc.
I don’t know how to create more pages in my plugin.

I hope you can help me cause I need to that for my job and I don’t know what else to do.

Thank you so much,

Gemma