Initially, the apartment costs the monthly rent, agreed in the rental contract. Added to this are the ancillary costs.
At Service charges include, for example, the costs for heating, water, the janitor, garden maintenance, waste water and much more. The ancillary costs must always be mentioned and precisely described in the rental agreement. Otherwise they are not allowed.
You usually pay a part of the additional costs monthly together with the rent. This is called Payments on account. At least once a year you will then receive a Final account. If the payments on account are lower than the final invoice, you will have to pay the remaining amount. If the costs are lower than this, you will be refunded the amount paid in excess. Check the final invoice carefully, it may be worthwhile.