Could anyone share the documentation of design rules of skywater?

You can get it in the following link: https://skywater-pdk.readthedocs.io/en/main/rules/periphery.html

Could you tell what is the unit of one square in magic when I use ' grid 1' command?

What's the width and length of ndiff? Is it specified in the doc? Also how to take contacts from ndiff, poly silicon? Does one always need to use upto metal 5 for taking contact?

What's the width and length of ndiff? you should determine the width and length of ndiff by yourself according to dimension of MOSFET. Is it specified in the doc? In the DRC doc, minimum width, length, spacing ..etc are determined, so you can take care while doing your layout. how to take contacts from ndiff, poly silicon? for ndiff and poly silicon you can use "ndcontact" and "pcontact" respectively.

@Binoy B: One square unit in magic depends on the snap spacing definition (and also depends on the specific technology). In sky130,

snap internal

sets the unit grid to 5nm.

snap lambda

sets it to 10nm, and

snap grid

sets it to whatever was set using the

grid

command.