LLMs generally work in one way. They get the prompt and give an answer. CLAUDE.md, system promp, rules, memory, tool defintions, mcps are different ways to prefix your prompt with extra information or context.

Skills, or plugins, are a way to inject less information until is needed (you can think about them as prefixing your prompt with “if you are asked about pizzas, add to context separate file pizza.md”).

What you could add to CLAUDE.md depends on what you’re doing. Generally it should be context LLM cannot infer relevant to all/most task performed in given project.