Each health insurance fund has a different structure of insured persons in terms of average income or health status. These differences are equalized as far as possible by a risk structure equalization (RSA) among all health insurance funds.
When distributing the financial resources, the surcharges and deductions are taken into account according to age, gender and the medical condition of the insured persons. Although health insurance funds collect contributions from employers or directly from insured persons, they transfer these to a state institution – the Health Fund (located at the Federal Social Security Office in Bonn) – on the same day and receive so-called allocations instead. The additional contribution of a health insurance fund is therefore not based on the financial strength of its own policyholders, for example, but on that of all those with statutory health insurance. Any excess amounts collected in this way are distributed to health insurance funds with below-average financial strength.
If the expenses for certain insured persons exceed 100,000 euros, additional funds are available from a risk pool. Further funds are made available through flat-rate preventive care fees and surcharges, which are intended to compensate for regional cost differences.
You can find more information at https://www.bundesgesundheitsministerium.de/risikostrukturausgleich.html