If one constructor calls another constructor which is the one that allocates the memory for the object

Let's consider following code:

class Auto{
    String color;
    int mileage
    public Auto (String color){
       this.color=color;
    }
    public Auto (String color, int mileage){
       this(color);
       this.mileage=mileage;
    }
}

which Constructor is the one that "constructs" the object and what does the other one do?

why is this allowed ? what are the benefits ? are there any cons ?