What is the best approach to learn a new topic in postgraduate Mathematics with a solid basis?
I'm about to start my thesis in postgrad Theoretical Physics but looking to learn a few topics for interviews on the side e.g. Combinatorics / Advanced Probability etc.
I am unsure if the best method is through books; online notes or a combination by following a specific course.
As long as you are using quality materials the content of what you are studying is more important than the source.
How did you most effectively study at the undergraduate level? Use those same study methods to improve your knowledge in the areas you want to improve. There is no right or wrong answer here and some combination of sources would seem appropriate.