Drupal has hook_mail function to send emails. This function is capable of sending email to a single user.
If we need to send an email to multiple user then we can write a function and call this function within foreach loop. This should be a simple and straight forward approach.
However there are lot of existing options which provide bulk email feature with multiple options like send emails taxonomy/user wise.
Here is a list of existing options, using which, we can implement bulk email functionality: