How Website Monitor works?

Internally, there’s a simple Google Script attached to the Google Docs sheet that does the monitoring.
The script invokes itself every 5 minutes, with the magic of Script Triggers, and then tries to fetch your website using URLFetchApp.fetch (a Google function similar to wget or curl). If the HTTP response code is anything other than 200, it indicates that there’s an issue with the underlying website.

How SMS Alerts Are Sent via Google

Google Apps Script can send email messages but the script employs a workaround for sending text messages.
It creates a regular event in your Google Calendar but with an SMS reminder  - the event is set to expire in 30 seconds and thus you get an instant text alert on your mobile. If you aren’t getting text alerts on your phone, please ensure that your phone number is associated with Google Calendar as detailed in this tutorial.