Explanation
The networkPolicy shown in the image is designed to block traffic from pods lacking the tier: api label, to pods bearing the tier: backend label. This is because the policy is set to matchLabels: tier: backend, and the ingress is set to - from: podSelector: matchLabels: tier: api. Therefore, any traffic that does not match these labels will be blocked.
References:
* Isolate containers with a user namespace | Docker Docs
* The mnt namespace - Docker Cookbook - Second Edition
* Container security fundamentals part 2: Isolation & namespaces
I hope this helps you understand the concept of networkPolicy and how it works with Kubernetes. If you have any other questions related to Docker, please feel free to ask me.

Explanation
Authentication is not a type of Linux kernel namespace that provides container isolation. Namespaces are a Linux kernel feature that provide isolation and virtualization of system resources for processes. They can be used to create isolated environments for containers that have their own view of system resources, such as process IDs, user IDs, network interfaces, etc. However, there is no authentication namespace in Linux. The types of namespaces that exist are mount (mnt), process ID (pid), network (net), interprocess communication (ipc), user ID (user), control group (cgroup), time (time), and user namespace (uts). References:
https://docs.docker.com/engine/security/userns-remap/,
https://man7.org/linux/man-pages/man7/namespaces.7.html

Explanation
Disabling the Docker service via chkconfig or systemctl does not upgrade Docker Engine CE to Docker Engine EE. Disabling the Docker service only stops and prevents Docker from starting automatically on system boot. It does not change or upgrade the version of Docker Engine installed on the system. To upgrade Docker Engine CE to Docker Engine EE, you need to uninstall Docker Engine CE and install Docker Engine EE following the official instructions for your operating system. References:
https://docs.docker.com/engine/install/linux-postinstall/#disable-docker-from-starting-automatically-on-boot,
https://docs.docker.com/engine/install/centos/#uninstall-old-versions,
https://docs.docker.com/engine/install/ubuntu/#uninstall-old-versions,
https://docs.docker.com/engine/install/debian/#uninstall-old-versions,
https://docs.docker.com/ee/docker-ee/linux-install/

Explanation
This is not a way to accomplish this, because adding all the resources to the default namespace is not a good practice for isolating applications in Kubernetes. According to the official documentation, namespaces are used to group resources into logical units that correspond to different projects, teams, or environments. Using namespaces can help avoid naming collisions and enforce resource quotas and access policies.
References: https://kubernetes.io/docs/concepts/overview/working-with-objects/namespaces/