Azure Security Center is a product used for threat protection and security management. It integrates with other Azure services and is offered through the Azure portal.
Azure Security Center is a unified infrastructure security management system that strengthens the security posture of your data centers, and provides advanced threat protection across your hybrid workloads in the cloud - whether they're in Azure or not - as well as on premises.
Microsoft Defender is an entirely different product from Azure Security Center. Microsoft Defender is for endpoint protection and Azure Security Center is for Cloud Security Posture Management and Cloud Workload Protection. You can, however, integrate Security Center with Microsoft Defender so that you have a unified view to monitor your servers and respond to threats. Microsoft Defender Security Center is the portal where you can access Microsoft Defender for Endpoint capabilities.
Microsoft 365 Security Center integrates Microsoft Defender for Endpoint (formerly Microsoft Defender ATP) and Microsoft Defender for Office 365 into one cross-domain security sweet. You can use it to set up email threat and alert policies and respond to threats across email and other apps in your organization.
The Microsoft Endpoint Manager Admin Center portal is for managing Intune and Mobile Device Management.
Cloud App Security is a Cloud Access Security Broker (CASB) used to broker access between enterprise users and the cloud resources they are accessing, and the portal is used for managing security on cloud apps.
Office 365 Security & Compliance is for helping meet legal or regulatory compliance standards in an organization, such as meeting GDPR or ISO standards and preventing data loss. There is a lot of crossover between the Office 365 Security and Compliance Center portal and the Microsoft 365 compliance center portal, and there are certain tasks that need to be done in one or the other because the Microsoft 365 compliance center is still under development.