How does the browser treat this stylesheet when the page loads on a normal screen?

The browser still downloads a stylesheet whose media query does not match, usually at low priority. But it does not block the first render.

"It never downloads it" is wrong, because the browser fetches it anyway in case the media query starts to match, for example when the user prints. "It blocks rendering like any other stylesheet" is wrong, because only matching stylesheets block rendering. This is why splitting CSS by media query can shrink the set of render-blocking files.