Another beginner question...

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.

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...