Access to Google Container Registry from bare metal Kubernetes

I created a test environment in a private cloud after reading Creating a single control-plane cluster with kubeadm - Kubernetes. Now I’m reading Concepts - Kubernetes and try to understand it. When reading Images - Kubernetes I’m asking myself where the access to Google Container Registry is configured in my cluster. Pulling images DOES work, I only want to understand why. Do I have a “Google service account”? Where can I see it?

thank you

Cluster information:

Kubernetes version: 1.17.4
Cloud being used: bare metal
Installation method: kubeadm
Host OS: CentOS 7
CNI and version: Calico
CRI and version: docker 19.3.8