The same way as it is done today. Plus the video feed from all cameras; plus an emergency signal to the trucking HQ; plus the exact GPS coordinates; plus the LoJack tag on the vehicle(s).
That is about the contents of the trailer. However if you are asking about a single vehicle, be it a car or a truck, or a tractor... they CANNOT be carjacked. You cannot threaten a robot with a gun, you cannot kick the robot out of the vehicle, and you cannot take over. Robot-driven vehicles are IMMUNE to carjacking. The most you can do is to stop them.
You are obviously not involved in the transportation industry. Margins are TINY in trucking, and few companies are going to be willing to spend the millions required to automate, given the uncertainty of freight arriving intact at the destination. The variables involved are beyond comprehension for a programmer or sensor studio.
Then there is the problem of repair. Refueling. And especially changing and dropping trailers. Desk jockeys think that those trailers stay with the tractor. Wrong.
In my profession, loading and unloading are especially hazardous. Yes, my insurance is expensive. I have my own DOT authority, and am a stand alone trucking company. I deal with Fed and State regulation compliance on a daily basis. For you to sit there and tell me you can program a truck to automate the entire process us both hilarious and sad. Mostly, it shows how little you know about trucking. I can still write compilers if I have to for many systems. Can you drive a semi? I’ll bet not. Until then, you should atay behind your desk in your office and let us deliver EVERYTHING YOU USE IN LIFE.