How HD textures are used in unity?

You don’t seem to quite know what your talking about.

HD just means textures like 2048 X 2048 or something like that.

They are used like any other texture. You import and apply.

using a lot of HD textures would quickly overwhelm the available memory in RAM and vid RAM however forcing the computer to pull the textures from the hard drive and basically making your game unplayable.