[venomsnake]

I have some macabre curiosity about how those people interact with the system at all. How do they identify themselves? And what actually prevents a person that just happens to be in the US from claiming he is citizen and getting id?

As I said where I live you get your id at 14, before that you are registered on the id of your parents. And it is impossible to interact with anything in official way without that id.

[waterphone]

They largely don't. See my other comment [1] for more specifics and sources, but the primary ID in the U.S. is a driver's license, and in poor urban areas people don't drive and so don't have an ID. They also often lack the documentation to make it easy to go get an ID even if they're able to—and in many parts of the U.S. the places to go and get an ID are only open a few days a month, which makes it nearly impossible for people with no car and who are often working multiple low-paying jobs to support their family to schedule going there. Someone who was very motivated to do so could likely manage, but it creates enough of a barrier that many don't bother voting as a result.

[1] https://news.ycombinator.com/item?id=12023285