The screen has no use whatsoever for any dpi number. Scanners do use dpi to create that image. Dpi is the method used by scanners to take color samples to convert inches of photo paper to be pixels for the digital image. Dpi means pixels per inch. Inches are on paper.
Printers also use dpi.
This dpi spacing establishes printed size. Dpi means pixels per inch. Inches are on paper. But show those 600 pixels on the screen , where there is no paper, and where inches have no meaning, and you simply see 600 pixels. One image pixel per screen pixel memory location, every time, regardless of the size of the screen.