From here, I could find that the linux nodes are automatically updated daily. Does the container runtime also getting updated to the latest one? Or do I need to manually updates the container runtime?
In AKS, Moby is used as the container runtime.
Release 2019-02-12 - Hotfix Release
CVE-2019-5736 notes and mitigation Microsoft has built a new version of the Moby container runtime that includes the OCI update to address this vulnerability. In order to consume the updated container runtime release, you will need to upgrade your Kubernetes cluster.
More information you can find here: aks-update
Overall information about aks you can find here: aks-core-concept.