Q: Does Groove work with iOS devices (iPhones/iPads)?
With iPads & iPhones that have USB-C ports built-in, just connect directly using the supplied USB-C cable and it will automatically be recognized and work.
*If the iPad or iPhone has a Lightning port, you need to use Apple's Lighting to USB3 camera adapter with an external power supply.
This is because Groove is a high-performance headphone amplifier and DAC designed to power the most demanding premium headphones. Though the newer USB-C iPads are able to supply enough power, Lightning-only iOS devices do not provide enough power for high quality headphone drivers with out this adapter.
See this article for more on using Groove with Lightning iOS devices:
How to use Apogee Groove with iPhone or iPad