Is this theory of how motion detection works correct?

That’s pretty much it. When you’re running a webcam in motion detector mode, you don’t want to end up with gigabytes of video of the back wall of your office. It’s been a while since I used one, but I think you can set up a detection frame rate, say, one shot every 500 ms, that the program can use to detect if anything has changed since the prior frame.

In addition, most cams take video at 30 FPS, not 60, although high-definition video might have a higher frame capture rate. For webcam use, 15 FPS is fairly common, too.

Thanks.