People who lives in the Tibetan Plateau are Tibetans, a 6 million of them. Traditionally, this plateau was divided into three regions; U-tsang, Kham and Amdo. People living in these regions use Tibetan language with their distinct regional dialects and practice same religion with same culture, the way they live.
When you look at a political map of the world, you will see Tibet is included in China. But when you look at a geographic map of the world, Tibet stands out with distinct borders marked by mountain ranges. This was how big Tibet was before Chinese occupation of the land since 1950.