When it comes to USB, there are 2 main roles: the Host and the Device.
iOS can act as a Host. Just plug in USB MIDI devices through the and it can be used right away. Many iOS devices have only the Lightning port. To plug in USB MIDI Device you would need an adapter such as the Lightning to USB 3 Camera Adapter.
iOS can also act as a Device when plugged into a macOS Host in IDAM mode. Unfortunately, the IDAM mode only works with macOS.