We need to understand whether by placing servers in MM (for example server A) , the existing alerts will be cleared and send clear notes to SNOW ticket?, so after removing severs from MM, alerts will be generated and auto-ticketed? please confirm.
No, the alerts are not cleared when you place servers in MM. They are exempt so that they don't affect the associated service. If you have the default alert filter you will not see those alerts because by default the filter setting includes the option "Hide Alerts for Services and Infrastructure Item that are currently in Maintenance." After you remove servers from MM, the alerts will have the Exempt status removed. It does not generate any new alerts.