Container-optimized OS (COS) is Google's linux distribution specifically designed for Google Kubernetes Engine container nodes. It is considered best practice to use this operating system in GKE over others such as Ubuntu.
In GCP Console -
In Terraform -
References:
https://cloud.google.com/sdk/gcloud/reference/container/clusters/upgrade
https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/container_cluster#image_type