Isn't it the same meaning to go to social security and buy social security?

Going to social security is the same as buying social security.

The unit should apply for social security within one month of employment and pay social security during the probation period. You can ask the unit to pay social security from the beginning of employment, or you can go directly to the audit department of the Social Security Bureau to complain about the unit.

Social insurance law

Article 58 An employing unit shall, within 30 days from the date of employment, apply to the social insurance agency for social insurance registration for its employees. If the social insurance has not been registered, the social insurance agency shall verify the social insurance premium it should pay.