Sending an email works with defining an email template (subject, text and possibly email headers) and the replacement values to use in the appropriate places in the template. Processed email templates are requested from hook_mail() from the module sending the email. Any module can modify the composed email message array using hook_mail_alter(). Finally \Drupal::service('plugin.manager.mail')->mail() sends the email, which can be reused if the exact same composed email is to be sent to multiple recipients.