C compiler was written in C language,that is true.But that does not say in what language first C compiler was written.I think first C compiler might have used assembly language instructions.And after that further compilers might have used this(or other compilers that was built the same way) compiler to compile the other newly build compilers.

There are cases when you need to define a constant pointer to a variable/object; for instance, when taking a function address, or when you want to protect a pointer from unintended modifications such as assignment of new address, pointer arithmetic

Contiguous memory is meant by continuous memory space in the available memory. so when we allocate a memory space contigously in our programming it is reffered to as contiguous memory allocation.

By using multiple structures means structure within a structure, we can achive inheritance in C.

itoa() is used to convert integer to string.

Because array index shows distance from the 1st element. Since the first element is zero distance away from itself, index starts from zero. The next element will be "index*scaling factor" distance away from 1st element. In case of integer array next elements will be  1*2, 2*2, 3*2, 4*2, .................bytes  away from the first element .