Has anyone understood what distinguishes Eldar, Noldor, Sindar, Nondor, Phalatrim, Galadrim, forest elves and green elves? They are all the same elves, only different names.

The same as the Tatars, Mordians, Chuvashis, Russians, Ethiopians, Papuas, Yakuta, Chinese, Swedes, Spaniards, etc. They are all the same people, but different names.