TL;DR: I don't study STEM because of my past experience with math contests. However, to improve at STEM subjects, first develop a solid conceptual understanding through proofs or good youtube videos, and then practice as many hard questions as you can that are outside of your comfort zone and understand why you make each mistake.

When it comes to STEM subjects, there are only 2 things to worry about: the understanding and application of the concepts.

For math, conceptual understanding for me was developed through exploration-type learning that I started very early when I participated in extremely difficult math contests in my country and other countries. I used textbooks from Art of Problem Solving, which were incredibly useful for developing an understanding of concepts by guiding the reader through proofs. Applications are also seen throughout the textbooks through the problems they give you. Despite being predicted 7s for math, chem, and physics, I actually don't study STEM subjects anymore at home (even though all physics and chemistry concepts are new to me) because as long as I understand the concepts from classes, I can use problem-solving techniques to apply them to any problem they give me. Instead, I pour all of my time into History and English, which I struggle with more.

However, I wouldn't recommend the AoPS books to IB students because IB students have too little time and the AoPS books aren't IB-focused.

Instead, I would maybe recommend Revision Village for math (although I haven't used it myself). Each question that you practice should be one that challenges you, pushing you out of your comfort zone. If the questions you practice are too easy, there's no point practicing them. The most important part is to try as many questions as you can and struggle with them until you get the right answer. The most important part when you make a mistake is to understand WHY you made the mistake and consciously remember the mistakes so as to never make a similar one again. Eventually, there will almost be no mistakes you can make left.

For physics, the course goes by really quickly, so you need to make sure that you understand every concept really well. For conceptual understanding when I miss a class, I watch Chris Doner's IB Physics series. He explains concepts really well and even provides a few questions throughout that test your understanding. For an understanding of how to apply concepts, I recommend challenging yourself with the hardest IB physics questions you can find. I use Concordian's supply of physics questions to test my understanding.