Puts are also automatically exercised. Your broker buys the stock for you and sells it to fulfill the contract. If you don’t have enough maintenance margin in the account, the broker may close your other position to cover it, or require you to deposit funds or charge you some sort of fee. Options and futures that close in the money are always automatically exercised, search it up. That’s one reason you never want to hold future to expiration, you will be required to take delivery of the oil or whatever the commodity is. Well some futures are cash settled. Oil is not.