Unity Atom's USB port

can you connect an iPhone to the Unity Atom’s USB port

No.
USB sticks or drives only.

Just use AirPlay.

Trying for a way to get high res. Into the home stereo

I use tidal connect which works great, but I have a large collection of songs on iTunes and I was trying to figure a way to get high-res out of the home stereo

Copy them to a USB drive and put that in the Atom USB port if you want the best sound quality.
Alternatively run a UPnP server on your network and point that at your iTunes library.

Try searching the forum. There was an identical thread here only week before last I think.
Similar advice was given.