A new “visit” is recorded when a visitor comes to the website for the first time and when they view a page more than 30 minutes after their last page view.
The “screen” analysis is based on the dimensions of the user’s screen, not the dimensions of the viewport through which they are viewing the site. On a desktop or laptop computer the viewport size may be different to the screen size (usually smaller) and therefore show a different layout to that assumed above. On mobile devices, the viewport is usually the whole screen.