It's called the roosevelt reservation, And it makes up a sixty foot wide strip that goes along the entire US-Mexican border. This land is entirely controlled by either the federal government, Or native tribes that live along the border. States have no authority or control over this land.
The primary reason the supreme court ruled Against texas Is because They were literally going onto federal land and putting up a razor wire on the border itself, Which is well within the federal government's jurisdiction along the border.