Apartment co-operatives almost always have mortgage debt secured by the land and apartment building. This is a mortgage payable by the co-operative corporation itself, seprate from any mortgage that the shareholder / resident might take out to fund the purchase of the apartment.
A portion of the shareholder's monthly maintenance is used to make payments on the underlying mortgage - but how much? Mortgages very hugely in their amount, interest rate, and amortization schedule. Some buildings have a mortgage of around one or two million for the entire building, i.e. only a few tens of thousands per apartment, which is not a significant level of debt or maintenance for the payments.
On the other hand, there are buildings with very substantial mortgages in the tens of millions of dollars, in the hundreds of thousands per apartment. These buildings generally have higher maintenance now, and are also at risk of large maintenance increases when the mortgage is due for refinancing if interest rates have increased.