The Raspberry Pi Zero USB port may be used as an USB serial port. Here is how I did it.
One will first add a device tree overlay (see Using SPI displays with a Raspberry PI). By adding
/boot/config.txt, one loads a device tree overlay which modifies the USB OTG configuration to make it more suitable to our needs.
We will then modify the kernel command line and add
modules-load=dwc2,g_serial at its end. This loads the serial gadget.
Starting a getty on the newly created serial port
Wait, there's more!
The Raspberry Pi Zero (and of course the Zero W) may appear as many other USB devices. See this Gist: https://gist.github.com/gbaman/50b6cca61dd1c3f88f41 and have fun!