They ask questions during the customs process, the typical “business or pleasure” thing. And if the agent senses any deception, you are sent with another officer that will interrogate you. What I don’t understand is why she was detained, immigration officer are allowed to deny entry to tourists. This whole ordeal could have been avoided if she was allowed to go back to Mexico and rearrange travel to Germany.
While I think the detention was completely unnecessary and cruel. It appears that her Instagram still lists dates she was planning to be in LA, right after a date range of being in Mexico, with a request for DMs under it for scheduling an appointment. It seems like this phrasing in the article of "accused of maybe violating her visa" could have disclosed that she looks to have a public declaration of her intent to work.