How To Setup Notifications of a Monitor?

First of all, from monitors list enter a monitor's edit dock and select "Notifications" Tab.
In this section you will see all users and notification types.
You can enable / disable with switches.

For Telegram and Slack Notifications, integration is required.

How Do I Stop Notifications During Routine Maintenance?

When your service is down for routine maintenance you can stop monitor (How to stop monitor?) or disable all notifications for users (on notifications tab).