The short version
OMNY, the MTA's contactless tap-to-pay system, has become the default way most riders pay across the subway and buses, and the older physical MetroCard is being phased out over time. For a visitor, this mostly simplifies things: your existing contactless credit card, debit card, or phone wallet works directly at the turnstile without needing to buy anything first.
What this means for a short trip
Unless you specifically want a physical card, or your card isn't contactless-enabled, you likely don't need to seek out a MetroCard machine at all. Just tap the same card or phone at the turnstile every time — OMNY tracks your rides and automatically applies weekly fare-capping, matching the value of the old 7-day unlimited MetroCard without you having to buy one upfront.
Where MetroCards still exist
Physical MetroCard vending machines are still operating in many stations during the transition period, and some readers may still be MetroCard-only in a shrinking number of locations — but the overall direction is clearly toward OMNY. Check the current status before a trip, since transit rollouts like this can shift their timeline.