There is no evidence that the Sumerians came from anywhere, and the prevailing scholarly opinion is that they were the indigenous inhabitants of southern Mesopotamia. Archaeologists have long argued that Sumerian speakers were the native inhabitants of Sumer, largely on the basis of continuity in material culture from the Uruk period to the arrival of the Amorites in the second millennium BCE. (We are much better informed about the origins of the Amorites, who seem to have migrated eastward from the vicinity of Jebel Bishri in Syria.)

Taking the opposing view, early philologists like Benno Landsberger thought they detected traces of a pre-Sumerian substrate in Sumerian texts, a view which has now been rejected but is sadly still often repeated in popular history books due to the enduring popularity of Kramer's The Sumerians. I wrote more about this in Mesopotamians and Sumerians.

Is your question a question of migration?

The question of where the Sumerian people originated and why the language appears to be a language isolate is a widely debated question with no definitive answer.

The broadest answer is that there have been hunter-gatherer tribes along the Euphrates for hundreds of thousands of years. Without irrigation communities could not grow beyond a certain size. With the advent of agriculture small villages could grow in size, expanding into what would eventually become the first civilizations.

There have been some archaeogenetic studies of the region. One such study on the genetic footprint of Sumerians is available at this link.