The courses in the Cloud-Native Development with OpenShift and Kubernetes Specialization are perhaps the best online training I have ever taken.
Even though the content does not excels deeper into each of the concepts, i think this is a perfect starting point for anyone who wishes to continue their journey with K8s. Both the theoretical and practical exercises were helpful in understanding the basics clearly and If you follow this course, make sure to refer to the guide book provided in the beginning as it contains some extra bits and pieces that will make your life easier in the long run
