You create an Azure subscription.

Security Center collects data from your Azure virtual machines (VMs), virtual machine scale sets, IaaS containers, and non-Azure (including on-premises) machines to monitor for security vulnerabilities and threats.

Data is collected using:

The Log Analytics agent, which reads various security-related configurations and event logs from the machine and copies the data to your workspace for analysis. Examples of such data are: operating system type and version, operating system logs (Windows event logs), running processes, machine name, IP addresses, and logged in user.

Security extensions, such as the Azure Policy Add-on for Kubernetes, which can also provide data to Security Center regarding specialized resource types.