How UI/UX design work for android apps

How does a ui/ux designer provide android-app designs? As far as I know a designer will provide a psd file to the developer. But android apps come in different screen sizes. So how does the developer get their required image/icons in different sizes?

Also does the designer need guidelines from the developer on sizes or ratios? Example - this icon needs to be of this size.

Or does the designer give just one psd and the developer converts them for different screen sizes?