Depends on the building but maintenance can cover a lot.
In my co-op it pays for:
-All utilities: heat, hw, gas, electric.
-Property taxes apportioned to your individual unit.
-The staff: 24 hr Doorman, porters, maintenance men, coop attorney, management company.
-Common areas: electricity and maintenance of elevators, hallways, laundry room, community room, landscaping of garden.
-Rainy day fund: account for big projects like roof replacement.
-Underlying mortgage - yes...