How to Create Your Own Vehicles
You won’t be able to actually start fabricating your own vehicles until you reach the Central Region past Port Knot City, and arrive in Lake Knot City to complete Order 23. Doing this will unlock the fabrication plans to create both the Reverse Trike and Long Distance Trike.
As for Trucks, you can hijack MULE trucks that dot the Central Region, but you won’t be able to create your own until you complete Order 35 at the Distribution Center South of Lake Knot City.