PTCL has setup fiber backbone, connecting whole of Pakistan, wateen is also doing same. Connection of internet is mainly through fiber optic undersea cables, Pakistan is connected to i think, 3 - 4 cables including SEAMEWE-4 etc. They land a place near Karachi from there they are connected to PIE (Pakistan Internet Exchange) from there data is transfers to country backbone fiber. As a backup satellite link is in Islamabad, it is used mainly as a backup as it has lesser capacity than fiber optic. I hope this answers your query.
I am doing Telecom Engg from FAST Lahore, currently in last semester.