How an option settles at expiration decides what you're left holding. There are two models, and SPY and its index cousin SPX sit on opposite sides — a distinction that catches traders who assume they behave the same.
SPY: physically settled
SPY options are physically settled. An in-the-money option at expiration delivers the actual shares: a long call auto-exercises into a purchase of 100 shares at the strike; a long put into a sale. So a winning 0DTE SPY call left open at the close doesn't pay you cash — it hands you roughly $74,000 of stock you have to fund, with all the margin and gap risk that follows.
SPX: cash-settled
SPX options are cash-settled. There are no shares to deliver — at expiration, an in-the-money option simply pays (or charges) the cash difference between the strike and the settlement level. No stock position appears, no auto-exercise into shares, no overnight funding problem. It's cleaner at expiration, which is one reason some traders prefer index options for holding into the close.
Same bet, different aftermath: an ITM SPY option becomes stock you owe for; an ITM SPX option becomes a cash number. Settlement is the difference.
What it means for you
Because SPY is physical, the discipline is non-negotiable: close your 0DTE SPY options before expiration rather than letting an in-the-money contract settle into stock. This is why NoVo makes closing a same-day SPY position a one-click move — on a physically-settled product, letting a winner ride into settlement is how a good trade turns into a margin call, so getting flat before the bell is the one call that's on you.