complications

You don't. Except through the weather app.

Check the weather on Apple Watch - Apple Support

Feature availability and data sources in the Weather app - Apple Support

Apple Watch Series 8 and Apple Watch Ultra have 2 temperature sensors—one on the back crystal, near your skin, and another just under the display. While you sleep, Apple Watch samples your temperature every five seconds. This design improves accuracy by reducing bias from the outside environment.  For more info, see:

Track your nightly wrist temperature changes with Apple Watch - Apple Support

Go to complications on watch face and select weather for where you want the complication to appear. Select outside temperature and hold the center of the screen.