The Zwift Companion app (the one with the turquoise icon) is like a remote control for Zwift that you can use pre-ride, during your ride, and post-ride.
Zwift Companion is primarily where you:
- plan your next activity, like signing up for a club event
- look at the results from your completed events
- give kudos to your cycling friends
But many also use it during the ride, as it is complementary to the ordinary Zwift app (with the orange icon)
Most people run it on their main phone.