The container runtime is a commonly used low-level component in container engines, but can also be used for manual testing. The OCI runtime standard reference implementation is runc.
The container runtime typically:
Consumes the mount point from the Container Engine (it can also be a regular directory for testing)
Consumes metadata from the Container Engine (you can also manually create config.json for testing)
Communicates with the kernel to launch the containerization process (clone system calls)
Configures cgroups
Configures SELinux policies
Configures AppArmor rules