Azure Monitor is a service that delivers a comprehensive solution for collecting, analyzing, and acting on telemetry (metrics and logs) from your cloud and on-premises environments.
These logs differ from the activity log. The activity log provides insight into the operations, such as creating a VM or deleting a logic app, that Azure Resource Manager performed on resources in your subscription using. The activity log is a subscription-level log. Resource-level diagnostic logs provide insight into operations that were performed within that resource itself, such as getting a secret from a key vault.
Microsoft Defender for Cloud is your central location for setting and monitoring your organizations security posture.
Microsoft Defender for Cloud is a Cloud Security Posture Management (CSPM) and Cloud Workload Protection Platform (CWPP) for all of your Azure, on-premises, and multi-cloud (Amazon AWS and Google GCP) resources. Defender for Cloud fills three vital needs as you manage the security of your resources and workloads in the cloud and on-premises, continually assess, secure and defend.
Microsoft Defender for Servers is one of the enhanced security features of Microsoft Defender for Cloud. Use it to add threat detection and advanced defenses to your Windows and Linux machines whether they're running in Azure, AWS, GCP, and on-premises environment.