Do I need sensors to see real time data (speed and cadence) on Strava Live? I would like to see real-time data (speed and cadence, no heart-rate for now) on Strava Live on my iPhone 7. Do I need to install bluetooth sensors on my bike or it just magically works? If it just works, is it reliable/accurate or I should still install sensors and let it get this information from the bluetooth sensors?
Strava (Premium) can determine your speed using GPS data, although GPS can be less accurate than a dedicated sensor because it jumps around a bit.

There's no magic way to determine cadence without a sensor, except maybe counting it in your head.