How to send email when my plugin is activated?

So there are two parts to your question:

1. How to run code when your plugin is activated
2. How to send an email

For item 1 I searched for 'wordpress plugin activation hook' and found https://developer.wordpress.org/plugins/plugin-basics/activation-deactivation-hooks/ which describes this hook so you can call your own fucntion when a plugin is activated:

register_activation_hook( __FILE__, 'pluginprefix_function_to_run' );

It is documented here: https://developer.wordpress.org/reference/functions/register_activation_hook/

For item 2, there are many examples of online of how to send email from PHP and/or inside the Wordpress infrastructure. As you are using this for a notification, I would suggest that you think about sending this notification with a very simple REST API style call instead. E.g. you could make a URL like yoursite.com/plugin/ping and your plugin could call that URL whenever it's activated, attaching extra information as required in e.g. the request parameters.

Note that however you send the notification, collecting identifiable information about people without their consent is immoral and also illegal in e.g. the EU under GDPR so you need to be very careful with what information you collect. You may want to consider licensing your software and having users agree to a license rather than what you're doing, however that subject is way off topic here.

I offer you an idea that I have already used something like this. I wanted to know the sites that my plugin uses, I created a cron job allow me to send me an email every week :

add_action('init', function() {
 $time = wp_next_scheduled('dro_cron_hook');
 wp_unschedule_event($time, 'dro_cron_hook');

 if (!wp_next_scheduled('dro_cron_hook')) {
    wp_schedule_event(time(), 'everyweek', 'dro_cron_hook');
 }
});

add_filter('cron_schedules', function($schedules) {
$schedules['everyweek'] = array(
    'interval' => 604800
 );
 return $schedules;
});

add_action('dro_cron_hook', function() {

 // Here you can send the email using wp_mail() and some additional data
});