Is there any documentation on how gyroscope parameters correspond to iTwin camera parameters?

[zhang-zimin]

we’d like to implement a feature where, as the phone moves, the device’s gyroscope updates the rotation of the iTwin model in real time

[zhang-zimin]

and could the iTwin viewer’s background come from the camera—i.e., I mean canvas background obtained via WebRTC ？

[pmconne]

If you can obtain an image from the camera you can use DecorateContext.setViewBackground to use it as the view's background.

iTwin.js doesn't have any built-in logic for controlling the camera via gyroscope - you'd have to put your own such logic into a new ViewTool.