I'm trying to figure out how "plug the GPIO pins into a computer's USB" to receive data from said computer. The results from my searches have been either too confusing or not helpful. Please be gentle, I have no knowledge of low level USB and serial. I may be good with computers, but I've never tried messing with this level of stuff. I have a pi zero.
You can make the Pi act as a USB gadget and impersonate a USB device.
Adafruit has a tutorial on how to do that -- or you can just keep reading for a summary.
Plug the SD card into another computer. Depending on the OS, you may see one or two new storage volumes. (If you see more, that's because you're using NOOBS. If you only have one OS installed, the rest of the tutorial will work.)
Find the volume with the files
cmdline.txt in it. Open
config.txt a text editor. If you're on Windows, open it in WordPad or Notepad, not Word.
Scroll down to the bottom and add
dtoverlay=dwc2 on a new line. Now save and close.
Here is where you have to decide if you want the Zero to act as a serial gadget or an ethernet gadget. If you choose a serial gadget, it will simply give you a login shell and you can go from there. However, you'll need to be to access a shell on the Pi some other way.
If you choose an ethernet gadget, it will show up as a network interface. This gives you a lot more flexibility because you can do things like VNC in -- which will give you a graphical desktop -- or you can share your computer's network connection with the Pi (Adafruit's tutorial talks about how to do that under "Ethernet Tweaks.")
If you choose an ethernet gadget, enter
g_ether instead of
g_serial in the next paragraph. You'll also want to create a file called
ssh (no file extension) on the SD card to enable SSH. You don't have to do that now, but it's convenient.
cmdline.txt. This file is one long line, so don't add any newlines. Go to the end of that line and add a space followed by
modules-load=dwc2,g_serial. Now eject the card and plug it into the Pi, but leave it off.
If you decided to go the serial route, you'll need to boot the Pi, log in by some other method, and run
sudo systemctl enable getty@ttyGS0.service to enable using the connection as a serial console, followed by
sudo shutdown -h now to power it off.
Now, use a micro-USB to USB cable to connect it to your computer (make sure to use the port labeled "USB"). The Adafruit tutorial goes into more detail on how to connect to your Pi from there.