In federal prisons, a quarter to more than one-third of prisoners are foreign nationals, and they are overwhelmingly illegals. The Federal Bureau of Prisons publishes official stats.
I do not know of a source that tracks state prisons for foreign nationals, and California probably does not collect or release such data. It is almost certain to be the same or more than fed prison percentages.
Give them a backpack and make them WALK across the country along abandoned rail tracks. Chain them to the track. Give them an airhorn to scare away the bears and mountain lions. It will take 6-9 months.