A Palestnian Arab is just an Arabised individual who happens to live in the former Palestian mandate and is not Jewish.
Back in the day, the land was partitioned into various sheikdoms and conquered by the Ottomans. The British got it, and carved out into Palestine to distinguish it from Egypt, the Jordanian kingdom, and French occupied Syria.
So, just as Israel is a modern construct, so is Palestine. The difference is that Palestine as a distinct region including Israel, has existed as a concept for 2000 years, just not as a country. Like "Ukraine" is not the same as "the Ukraine", one is a country, the other is a region.