How do I understand physical pixels and css pixels?

I do not understand this passage:

Later, with the development of technology, mobile device screen pixel density is getting higher, starting from iphone4, Apple introduced the so-called Retina screen, the resolution increased by one times, to become 640x960, but the screen size has not changed, which means that the same size of the screen, the pixel is more than one time, then, A CSS pixel is equal to two physical pixels.

There is also a factor that will cause the change of PX in CSS, that is, user zoom. For example, when the user enlarges the page by one time, the physical pixels represented by 1px in the CSS will also increase by one time, whereas the physical pixels represented by 1px in the CSS will be reduced by a factor of one time.

Why did the 1PX corresponding physical pixels in the CSS increase after zooming in? Should not have been reduced.

Explanation: The answer is helpful to the person, has the reference value 0 The answer is not helpful, is the wrong answer, Irrelevantly replying

Only discuss the width or height of the area by themselves.
When the user enlarges the page (the finger slides outward and sees the element become larger), the physical pixel represented by the 1px in the CSS increases by one times.
The 1.100px div will always be 100px if zoomed out, because this is the element unit.
2.200 physical pixel devices, no matter how zoomed in or out, are always 200 physical pixels because the screen hardware is dead.
3.200 physical pixels Just drop the two 100px Div, 1px=1 the physical pixels.
4. When the page is enlarged, 200 physical pixels can only see 1 100px Div, ask 1px equals how many physical pixels.