How do I know which research method fits which question?

You can run mixed methods now, which means the tools are in your hands. What still trips you is matching the tool to the question, and that is a subtler skill than the methods themselves. It is the difference between knowing how to use a thing and knowing when. That judgment comes slower, and it is normal to still be building it. Here is a simple anchor. Ask what the question is really after. If it wants to measure how much or how many or how often, that leans one way. If it wants to understand why or how something is experienced, it leans the other. Most confusion comes from skipping this and reaching for a familiar method out of habit. Start with the question, not the tool. For your next study, before choosing anything, write the research question in one plain sentence and then ask what kind of answer would actually satisfy it. Let that answer point you to the method, not the other way around. Do this every time and the matching will stop feeling like guesswork. The methods you already know. What you are learning now is judgment, and this habit builds it.