Vector diagram, not diagram, what do you mean? When you want to make a poster, why do you want to save it after PS is completed?

It is right not to see clearly after zooming in.

Bitmaps consist of pixels.

Each resolution has its own unit, for example, the resolution of the screen is 72 PPI.

(pexl pixels per inch per inch)

Bitmaps are made up of these small pixels, just like cells. They are very small and square, so when the image is magnified infinitely, you will see small squares.

A vector diagram, also called a graph, consists of paths or curves. Every figure is composed of many mathematical formulas, so it will not be blurred no matter how big it is.