So back in February, I discovered (https://twitter.com/MishaalRahman/status/1621194700790054914) code changes in AOSP that hinted at a new "DeviceAsWebcam" service (which upon re-reading the code changes I now believe to be a separate system app) that "turns an Android device into a webcam."
Android devices that support the UVC gadget mode (ie. the kernel is compiled with CONFIG_USB_CONFIGFS_F_UVC=y) can be set up to send camera frames to a /dev/video* node that a host device supporting UVC webcams can read from.
A dedicated DeviceAsWebcam app - which I should note has not yet been shipped on any Pixel builds - is what, I believe, would be responsible for actually using Android's Camera API to forward frames.
Only system apps-like DeviceAsWebcam-would have permission to R/W data to the /dev/video* node. In fact, the USB configuration file for all Tensor & Tensor 2-based (but not SM7250) Pixels already sets up the UVC function. Here's a snippet from init.gs101.usb.rc/init.gs201.usb.rc: