Max USB Webcam Resolution and Frame rate
It depends on the external camera provider specs.

From External USB Cameras  |  Android Open Source Project


The USB camera HAL process is part of the external camera provider that listens to USB device availability and enumerates external camera devices accordingly.





Webcams generally support USB video class (UVC)
drivers and on Linux, the standard Video4Linux (V4L)
driver is used to control UVC cameras.

Probably, what's quoted is typical or frequently found values but if you want specific information you would need to dig into the code of the device + camera provider.
As asked in the comments maximum limits can be gleaned from

the camera specs and codes

format used  (for example UVC format itself has multiple variations - see linked wiki above)


but whether the device uses full capability or not is the important point.
You would find tons of rooted means and Magisk modules that improve the camera performance with the same device and camera - it's only possible because ROM didn't build in those features.