In Short, from about 250,000 around the end of the 19th century, many of them bedouins, the Arabic population grew to about 1,250,000 in 1948. The Palestinian claim that they are the ancient population of the so called Palestine has no ground.
Winston Churchil, said in May 22, 1939 that the Arab immigration to Palestine during the British Mandate was so large that their numbers grew in such proportion that even if all Jews immigrated to Palestine they could not reach that number.
Franklin D.Roosevelt, said in May 17, 1939 that the Arab immigration to Palestine since 1921 was much greater that Jewish immigration.
A significant part of the 1948 Palestinian refugees were first or second generation illegal immigrant workers.
Arab Immigration into the Coastal Plains of Israel (the Sharon) During the British Mandate.
That conclusion is disputed but I will cede the point. It still makes no difference. The vast bulk of the Jewish population is also from recent immigration.