Because only the municipal (district) and county-level administrative health bureaus (epidemic prevention stations) have the right to apply for health certificates.
Other departments, institutions and hospitals have no right to handle it. Even if they do, it is invalid and cannot be checked. (Used for internal support of the unit at most)
You need to bring your ID card and a letter of introduction from your work unit. Be careful not to eat or drink water in the morning, because you need a blood test.