How do services work to determine if a page is responsive?

Question

Adaptive design

How does, for example, Google's service for checking page adaptability work:
https://search.google.com/test/mobile-friendly ?
How does he, for example, measure the distance between interactive elements in order to understand whether it is sufficient or not and give an error / say that everything is ok?
On what PL and using what technologies / tools can you do something even remotely resembling such a service?

Answer 1

headless browser and collect computed-css properties of all object borders.
Next is geometry.