Another beginner question...

Have you tried Bluetooth wireless streaming from your iPhone to the Teac AI101-DA DAC integrated amplifier... Might be a little easier to use than wired USB...  There is no analogue out on the newer iPhones...  They are all Bluetooth wireless now for earbuds and external speakers...

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.

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

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.

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...

“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.

That's what updates are for.

It might work but I'm Android and most dongles for my wife's iPhone do not work.
Apple tries hard to keep you from doing anything. If it works today probably not tomorrow.  Beware

My Phone is Android - I tried that with otg cable and USB pro audio player but it does not work with my W4S dac2
 May be with other Dac

I believe I read somewhere there was a "Lightning Dongle"that plugs in & sends a digital signal via cable..Sorry I can't be more help...