show temperature on home screen iphone 8

You can view the weather/temperature as a widget on the widget screen—swipe right from the first home screen. For more information about widgets, please see this support document: Use widgets on your iPhone, iPad, and iPod touch - Apple Support

Not with the iOS builtin weather apps, but there are other 3rd parties app that can show temperature as a bubble on its icon.

Also you can ask Hey Siri, what's the temperature?