During the specified period of the trading session (American, Asian-Pacific, European), the market makers must continuously maintain two-way futures quotes, observing the minimum volume of own orders agreed with the exchange and the spread between bid-ask quotes (widening of spread for periods of increased volatility is possible). As counterparties to each transaction in terms of pricing, market makers must take the opposite side of your trade. In other words, whenever you sell, they must buy from you, and vice versa.
Why do market makers act as counterparties for most orders, but not for all? The rest of the transactions may temporarily be accumulated by other market participants, both with speculators and hedgers; however, over time, all open positions will pass on to market makers, when one of the parties of a transaction, in which the market maker does not participate, decides to close the position.