If you want to get deeper (or wider?), you can get a textbook (e.g. Introduction to Algorithms by Cormen) or a university course like MIT 6.006 but obviously those will contain a lot more than just this topic.
The basic concepts of Big O should be well explained on YouTube. If you are looking at calculating time complexity for larger program flows, then you need to learn about mapping the process flow, figure out the complexity of each piece of that process and how it is invoked, then create an algebraic representation of that information.