They just ask most people if they are a citizen and if they look white enough CBP waives them past. If they don't CBP pulls them into secondary where they have a sniffer dog that alerts on their car allowing them to search the car and it's occupants at which point they will check their ID and run it through their system.

Personally I refused to answer, set the parking brake, turned the car off, and removed the key. They told me to go to secondary and I just didn't say anything and kept looking forward. Eventually enough traffic backed up behind me on the freeway they just waived me through to get traffic moving again. I know the courts have found it to be legal, but I don't care. I'm not going to comply. If no one complied they wouldn't be able to get away with this shit. Unfortunately most people don't care.