When training inside, all you need is your iPhone connected to your sensors to be able to train. Upon completion of your workout, the data will sync with Garmin Connect as long as you have RideSync enabled. You head unit is not needed for this process.
When riding outside, on the other hand, you have the option to push your workout instructions to your head unit to follow along with out on the road. You can learn about the process here:
In this case, you are not using your iPhone to train; you’re just using the head unit.
I hope this helps!