Hi 一波 陆,
Welcome to the Microsoft Q&A Platform! Thank you for asking your question here.
Microsoft Defender Antivirus (formerly Windows Defender) is built-in and enabled by default on Windows Server 2016 and later. The Azure VM Antimalware extension can still be added to these VMs to apply custom configuration policies for Defender Antivirus. The extension does not deploy a new antimalware service.
For more details, please refer to the below documentation:
Microsoft Antimalware Extension for Windows
Default and Custom Antimalware Configuration
Azure Virtual Machines security overview
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