How the big plants do ready-mix concrete? What materials and other things they use? I have seen those big concrete trucks in California.

Redi-mix is a combination of concrete, sand, crushed stone and water.
Concrete is ordered in "yards" not square feet.

When the truck is at the concrete plant, it pulls under hoppers. There the proper measurement of materials is added to the truck depending how much is needed for the delivery. When you see the drum of the truck turning, this is the mixing porcess as it goes to the job site. They also need to make sure the drum is turning so it doesn't set up if they have a ways to drive.

Chemicals can be added to the mix at the plant to increase set time, decrease set time & prevent the concrete from freezing in cold climates.