When trading a single leg option (eg: sell put, buy put, sell call, buy call), there are 2 things that will happen at option expiration: either the option expires worthless (out of the money), or the option gets exercised/assigned (in the money).

If the option expires worthless, no transaction will happen. The option will simply disappear from your portfolio.

If the option expires in the money, it will get exercised. The brokerage (in this case, Moomoo) will execute the transaction automatically.

What is in the money (ITM)?

For put option, in the money means the stock price is less than the option strike price, hence it makes more sense for put buyer to exercise the option and sell their stock to the put seller rather than sell to market.

Meanwhile, for call option, in the money means the stock price is higher than the option strike price,