South Punjab is known as Saraiki Riyasat. They are the indigenous people of the area. Of course small numbers of punjabis are also indigenous in the villages and farmlands, but after partition, there was a bigger influx of punjabis that moved to the area. And that is why you might be confused about all the punjabis in south Punjab. Saraikis are also linked to punjabis, but they are also linked to Sindhis, Baluch, and Pashtuns. Think of them as the base or core of Pakistan. Or at least middle Pakistan.