I've heard that testing should be based on application behavior and how users interact with the product, which makes sense. What's extremely unclear to me is when is a feature obviously working to the point where you don't need to test it? For example, I see why testing the results of an API call is important even if manually testing it in Postman/Insomnia/the browser yields expected results. But I'm confused by something like testing of what gets rendered in React. How do you decide what portions of your code need to be tested?