I agree with the above post, this is a massive field and most here didn’t have a structure for learning either. The way most of us learn is by running into a problem and then researching solutions to that problem. In most cases your searching for a solution to the problem SHOULD result in a better understanding of how whatever service/application/OS you are working on works and operates. Something I always do with my notes is add a lessons learned section, this should help with the above issue as well to help retain some knowledge.