Check your webcam in half a minute before a video call. Alongside the live preview you get the resolution, aspect ratio and frame rate actually coming in, so you learn not just whether the camera turns on but whether the quality is right. You can also grab a snapshot.
This usually means another app is holding the camera. Fully quit Zoom, Teams and similar programs and try again. Also check for a physical shutter or camera kill switch on your laptop.
No. Both the preview and any snapshot stay inside your browser. Nothing is kept anywhere unless you download it.
It's measured by counting frames as they actually arrive, but browsers that don't support per-frame timing fall back to the value the camera reports about itself. In that case it's labelled "reported by camera".