Why does social security need file custody?

It's not as complicated as you say. If you work in a company and pay insurance, the company will have your recruitment registration form and contract. These are your papers. According to the regulations, after you resign, the unit must transfer your file to the employing department or individual, and then you can transfer your file to other units or file storage centers for renewal. Calculate the pension according to your insurance file when you retire in the future.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.