The USB-A's are for thumb-drives. The USB-B is for computers. The USB-A's labeled ACCESSORY POWER is so you can power external devices like bluetooth receivers or other devices that operate on a 5V power supply.

You would not be able to plug a phone directly into the unit unless you have a headphone output.

When the phone app is released, if you have Spotify Premium, you will be able to directly send Spotify to the RR2160MkII via the app.