I'm interested learning more on variational calculus, I found it useful in addressing some PDE-related problems, but I never had a formal course on it.
There are people who think it is appropriate to merely suggest reading a book, but obviously if people magically learned everything by picking up a book, universities and professors would never have existed, we would only ever have libraries and nothing more, so such responses are off-topic unless they are more constructively integrated into a holistic solution.
Simple web series are too colloquial, but research papers are too hastily technical without explaining the context and steps. I see there is room in the middle between these, but I have not seen any particular organization take the steps in creating it so far.