[Figs]

> lots of JS libraries expect things like viewport and window size information, I wonder how Google is achieving that.

Just plug in common screen parameters (e.g. 1920x1080, 1366x768, ...) and analyze it as if it were the result you'd get by default with Chrome on such a screen, I would imagine.

[tracker1]

Same goes for user agent spoofing (to some extent). You can imagine most of the stuff when you use the chrome dev tools being done without actual user interaction.