Another beginner question...

“My DAC amp has a USB-B port in the back.”

First thing, the USB-B port on your DAC designated as digital input or for service updates?

The lightening to USB-B cable (if you can find one) does not support audio or video output, so your iPhone will not output any audio to your DAC.

It might help if you gave the make and model of the equipment involved in your question... Someone might RTFMs and give an answer...  Or give other alternatives...

Teac AI101-DA DAC integrated amplifier, Sony bookshelf speakers.  Super fast WIFI connection from a router.  Currently using YouTube Music Premium and Amazon Music Premium services.  I can go optical from my TV into the amp, but it would be nice to steam without using the TV.

...and an iPhone, which can control the TV YouTube Music and Amazon Music apps.

You can connect a music player like an iPod with a minijack I don’t think iPhones have those anymore. You can connect a laptop to the USB. You can try an adapter for USB not sure if it will work on the phone. You could get a streamer and connect either optical or USB. You can use bluetooth to connect the phone.