I'm really curious to know what are these holes on this mobile PCB:
Are these vias? I doubt they are vias. If these are not vias then what are they? What's the usage of these holes?
What you have circled are vias. At that size they're commonly called microvias. The technique being used is called via stitching. Many vias are tying the copper pour on the outer layer to an internal ground plane. This is an essential technique for EMC in RF and other high speed circuits.