Mexicans (and other nationals) become illegal aliens when their feet touch U.S. soil, if they have come here any way other than that prescribed by the laws of the United States.
From that moment on, the are subject to removal until they leave.
1 day, 1 week, 1 month, 1 year, 1 decade, or 50 years later, they are subject to removal and extreme loss of assets and their developed life style.
Foreign nationals, don’t do it. Don’t subject yourself to what comes next, when you come to the U. S. illegally, and then invest yourself in developing a life here.
It can go poof any moment.