How do insurance companies know whether they are sick or not before taking out insurance?

In the Internet age, a health code knows your recent activities. Going to the hospital for a physical examination or seeing a doctor with an ID card or medical insurance card will leave traces. As long as the hospital cooperates with the insurance investigation, it will definitely find out. Suppose you go to a private medical examination institution without your ID card or medical insurance card, even if you leave traces, the medical examination institution may not necessarily tell the insurance company the information. In a word, insurance investigation is limited and not so powerful.