The cgroups code is in the container engine selected, not in k8s. K8s take care of running containers and talks with the runtime using CRI. CRI is an API to let any container engine interact with kubelet. Kubelet is a piece of kubernetes that sits on every node and make sure that all pods are running as expected.

Take a look at libcontainerd for docker as container engine and cgroups package for containerd

Regards.