How to do a countdown on iPhone without apps

Ask Siri how many days until your date, or add Apple's How Many Days Until shortcut from the Shortcuts app gallery. Both answer on demand. What they cannot do is keep the countdown in view: no widget, no Lock Screen count and no alerts, which is where a countdown app earns its place.

The genuinely app-free options

Ask Siri. "How many days until 25 December?" gets you a number on the spot. Quick, and gone as soon as it is spoken.
The Shortcuts gallery. Open Apple's Shortcuts app, search the gallery for "days", and add How Many Days Until. Run it whenever you want the count, or ask Siri to run it.
Calendar and Reminders. The Calendar app shows the date, and a Reminder can ping you when it arrives, but neither counts down.

If you just need the occasional number, honestly, Siri is enough.

What you give up without an app

Every method above is on demand: you have to ask, every time. There is no countdown sitting on your Home Screen, nothing under the clock on your Lock Screen, no alert a week before so you can plan, and nothing ticking through the final hours.

If you decide the date deserves more

That standing countdown is the whole point of a countdown app. Event Countdown is free, needs no account, and puts the days-to-go on a Home Screen widget in about a minute, with a Live Activity for the final hours. Try it against Siri and see which one you look at more: how many days until Christmas.