The kubelet, in OpenShift Container Platform 4 and later, uses the bootstrap certificate located in
/etc/kubernetes/kubeconfig to initially bootstrap. This is followed by the bootstrap initialization process and authorization of the kubelet to create a CSR.
In that process, the kubelet generates a CSR while communicating over the bootstrap channel. The controller manager signs the CSR, resulting in a certificate that the kubelet manages.
This bootstrap CA is valid for 10 years.
The kubelet-managed certificate is valid for one year and rotates automatically at around the 80 percent mark of that one year.