The definition of "border" is surprisingly vast too -- if you're within 100 miles of any "external boundary". Two thirds (!) of Americans live within this "border" area.
This is a bit of an exaggeration, which has frequently been de-bunked. In brief, if you didn't recently cross a border, then immigration officials have no special powers within this zone.
There is, however, a "functional equivalent" of the border in every international airport that grants ICE these powers over arriving citizens (which makes sense).
https://www.aclu.org/other/constitution-100-mile-border-zone