Full first page on every size - how do you achieve this? (example included)

I think the Zapier front page is awesome: https://zapier.com/

But i'm wondering, how do you achieve that the first page is always the exact height on different sizes?

What's the easiest trick to achieve this ? Do they use something like backdrop?

The background-size CSS property can be set to cover, which scales the background as the page is resized.