Can I use USB-only power to run a stepper motor?

Post links to datasheets or product pages for all the hardware you mention so we actually know what you have please - we cannot guess this stuff and all details matter typically.

Also a motor shield can't drive a stepper - you use a stepper driver for a stepper. Stepper motors don't have a voltage rating, they are current driven.

No, generally stepper drivers require higher voltages like 12/18/24V. You should never share a motor supply with digital electronics anyway as motors tend to pollute their supplies with massive noise spikes and drop-outs.

I read a lot of contradicting information here and elsewhere...

This is my stepper motor: NEMA 14 Motor - Joy-IT

This stepper motor is connected to the M1/M2 Pins at the Adafruit Motor Shield v2.3

What do you mean by stepper motors don’t have a voltage rating?

Can you recommend external power solutions, which can go into the Adafruit power terminal?