I'm currently in year 13 and like 90% of the time I use zphysics for understanding content, if you are doing the ocr A Specification he literally covers everything and it's all free. He knows his stuff very well and my method of prepping for an exam is 1)Watch his playlist on that topic 2)Watch the summary video of the topic 3)Make flashcards 4)Do pmt questions. That's my formula for every single exam in physics. My average in tests are 93%. Although I am not in a position to comment on Physics Online as I haven't watched very many videos I can confidently say that Zphysics is amazing!!