linked page not working with my wordpress plugins

We used this code to create a menu page which loads mypluginpage.php
and it works fine.

I can access it from something like https://mysite/wp-admin/admin.php?page=myplugin

<?php

// Add menu
function myplugin_menu() {

    add_menu_page("myplugin", "my plugin","manage_options", "myplugin", "mypluginpage",plugins_url('/myplugin/img/icon.png'));
}

add_action("admin_menu", "myplugin_menu");

function mypluginpage(){
	include "mypluginpage.php";
}

Here is my issue: Inside mypluginpage.php below, I have a link that should take me to welcome.php within admin page

<?php 
wp_head();
?>

<a href="<?php echo admin_url('welcome.php'); ?>">welcome page</a>
	
<?php
wp_footer();

I thought am supposed to have something like this link within the admin
https://mysite/wp-admin/admin.php?page=welcome

but I got direct to link below when I click on Welcome Page link

https://mysite/wp-admin/welcome.php

which leads me to page not found.

I need to have welcome.php page to called within admin. any help will be appreciated..