The Palestinians were the people resident in Palestine. They are the indigenous people of Palestine. That is all. The reason the mandatory language is
"those who were habitually resident in Palestine" is because they were dissolving the Ottoman Empire, which had been the only political structure in Palestine for literally hundreds of years. But the people weren't Turkish. As genetics has proved, they are the descendents of Canaanites and Sea Peoples, Philistines, Assyrians and Jews, Romans, Arabians, Crusaders, many different migrations and influences. But there has always been a people present continuously on that land. They are largely Sunni Muslim, but 20 - 30% (depending on location) were Christian, and others were Druze, Bahai, Armenians, indigenous Jews, Circassians, etc. All these groups are Palestinian, most but not all were Arab.
https://en.wikipedia.org/wiki/Palestinian_people
Hee are some sources on the topics that are being discussed here:
The Palestinian right of return - legal and moral basis:
https://en.wikipedia.org/wiki/Palestinian_right_of_return
And here are a couple of articles on why they left:
https://en.wikipedia.org/wiki/Plan_Dalet
https://en.wikipedia.org/wiki/Deir_Yassin_massacre
And last and saddest to me, the very many villages, some going back to Bible times and earlier, that were destroyed:
https://en.wikipedia.org/wiki/List_...epopulated_during_the_1948_Palestinian_exodus
You can click on links to read the story of each individual village. These stories are compiled from not only Palestinians who were present, but also the stories of members of Haganah, Irgun, etc. about their own roles in each case. They don't deny what they did. And these villagers had very few arms. Virtually unamed and, as you will read, in many cases totally non combatant.
In the case of Deir Yassin, a village on the outskirts of Jerusalem, the Arab population had agreed with the neighboring Jewish town to support and protect each other, a mutual defense pact. So the villagers refused to allow Arab militias into the town to protect them, believing their solidarity with their Jewish neighbors would prevent them behind harmed. When the attack came, not a shot was fired by the villagers in resistance. The Jews from the neighboring town tried to honor their agreement and ran over, running through the streets screaming "No! No! Stop! Don't kill them!" ...but to no avail. Irgun, Lehi, and Palmach units were responsible for the massacre. Afterward the survivors were paraded through streets of Jerusalem and elsewhere in trucks for several days, telling Arabs to leave before the same fate would befall them. No one really knows what happened to the "survivors" so it is possible that they were executed after being driven around, but I don't think there is any proof of that yet. At any rate, no one has turned up and said "I survived the post-Deir Yassin truck ride." It seems fitting somehow that the buildings are now part of an Israeli insane asylum.