Flashing TWRP to your device via fastboot, and then booting to TWRP allows you to transfer files to and from the phone with a USB cable.
Thank you very much.
However, don't see anything inside of my current TWRP recovery that lets me copy files to and from my PC. The only option I see is "sideload", which is used specfically for flashing zip files, not copying arbitrary data to or from the PC.
What am I missing?