Confused in Retinas. There is a retina for iPhone, there is for Android. Factor @2x, @3x and @4x. What pix/inch resolution should I use for retina design?

Question

Mobile development

Confused in Retinas. There is a retina for iPhone, there is for Android. @2x, @3x and @4x
multiplier What pixel/inch resolution should I use for retina design?
In theory, it is better to do with a margin of @4x.
But the original resolutions are different for iPhone and Android.
In summary:
What is the original pix/inch resolution?
What is the final retina pix resolution with a margin (ie @4x) ?

Answer 1

Modern design is done in vector.
Then it is rendered at the required dpi.
Raster graphics are taken in the maximum available resolution, then adapted to the required point density.

Answer 2

Thanks for the helpful pictures.
But you need specific resolutions in pixels / inch.