Yes you are correct but they are not limited to border crossings only. Yeah i live in the USA bro bro. They are a officers of the law so they can still stop someone if they have "probable" cause or see you doing something wrong. CBP agents may search for noncitizens on any “railway car, aircraft, conveyance, or vehicle” that is “within a reasonable distance from any external boundary of the United States.” A reasonable distance is considered 100 air miles from any external boundary of the United States except if they witness a crime they can go further. That being said CBP has roving border patrols within 100 miles or more (if they have probable cause to go the distance). Roving CBP patrols are legally allowed to pull over anyone and ask about their citizenship or immigration status of they have "reasonable suspicion" of unlawful activity. Just to clear this up "unlawful" could be anything from running a redlight or stop sign or even a broken tail light. But we all know how the cops are in the US.

"LET ME SEE YOUR HANDS OR IM GONNA FUCKING PUMP YOU FULL OF LEAD, YEAH OK LICENSE AND REGISTRATION YOU PUNK" reaches for registration "BANG BANG BANG BANG BANG BANG BANG BANG" "I TOLD YOU TO KEEP YOUR FUCKING HANDS IN THE AIR YOU DEGENERATE PUNK, YOU SEE WHAT I HAD TO DO BEACAUSE YOU DIDN'T LISTEN TO DIRECTIONS" FULL BODYCAM FOOTAGE OF THE MURDER AND THE OFFICER GETS A SLAP ON THE WRIST WITH A PAID VACATION