My gawd. No, the Palestinians do not come from surrounding countries. Genetic testing has proved they are the descendants of the Jewish population. Here is the picture, which I am reposting:
The Romans crushed the Jewish state in A.D. 70, depopulated the cities, took many people as slaves to Rome, but the majority escaped to the countryside and continued their resistance .... Masada, Bar Kochba, etc. Of course there were other peoples there at that time, too, it wasn't 100% Jewish, but pretty much.
The synagogue tradition was developed, and some of those synagogues morphed into churches as more and more Jews converted. By time of Constantine, Palestine had a Christian majority, descended from the Jewish population.
At the time of the Muslim conquest there were many conversions, and a little intermarriage, a new language and a new culture that mixed in with the old, but no wholesale migration took place. The bedouin Arabs who were the nucleus of Islamic expansion, were a very tiny population anyway and couldn't have staged a large migration.
Later admixtures have got to include the Crusaders, Turks, etc. To this day you can see blonde haired, blue eyed Palestinians, and black African Palestinians.
But the population of Palestine remained continuous throughout. Note we have father and son, mother and daughter, all the way back to Canaanite times. And Palestinians are about 80% descended from those who were Jews in Roman times. By the way, modern Jews are descended about 72% from those who were Jews in Roman times. So Palestinians are more Jewish than the modern day Jews are? No, I don't think that is the point. The lesson is about demographics and migration.
And by the way, of course straight through there was a population of Jews who never converted. These were true "Palestinian Jews." The pattern is similar to that of the Samaritans who were left behind during the Jewish Babylonian exile, and remain there today in fact, still practicing a very old form of Judaism that existed before the Babylonian exile.
And this is what genetics has now proven. There is no place for other theories at this point.
It is interesting to see the continuity. Someone made the point that the same patterns that appear on Palestinian women's embroidered dresses are seen in Canaanite handcrafts. All over you will recognize the place names of the Bible. Cana, where Jesus turned water into wine, became Kafr Kana, etc. Most of the Christian traditions native to the Levant and Palestine .... Maronite, Syriac, etc. .... use Aramaic as their religious language. It was the language spoken by Christ.