They get 'due process'.
Are you a citizen? No.
Do you have a visa? No.
Do you even have a stamped passport? No.
Bye.
Discussion
Is Mick a citizen? No (they can lie)
Does Mick have a visa (obviously not)
Do you even have a passport? (Most people don't).
Mick is deported to El Salvador and can't even come back to his own country because you are now a criminal and criminals can't get visas.
What is your working definition of a “citizen” then, just someone who happens to be living within a designated area? Or is there a due process for rights and responsibilities of belonging to a State, with all its pitfalls and benefits?
Deporting someone is one thing. Shipping them directly to a prison is something else.
You don't need due process to enforce immigration policy.
You do need due process to deprive a human being of liberty.