I confirm that the USB on the AVR X3600H receiver is for storage use only. It doesn't act as a DAC input.
To get hi-res Apple Music a USB DAC is needed. DACs with a USB C input should work directly (iPhone 15 Pro -> USB C cable -> USB C DAC). I say "should" because there're many DACs on the market and it's difficult to say which are fully compatible.
DACs with a USB B input should need a USB-C to USB Adapter – USB-C to USB Adapter - Apple (iPhone 15 Pro -> USB-C to USB Adapter -> USB A-B cable -> USB B DAC).
In my experience, some USB DACs are picky with power consumption meaning that in some cases a powered USB hub is needed in the iPhone-USB DAC chain (iPhone -> USB Adapter -> Powered USB Hub -> USB DAC).
With my iPhone 14 Pro (featuring a Lighting port) I can use a Topping USB DAC without any powered USB hub. In my case, of course, I use a Lighting to USB Adapter.