programming question about the use of cstring with array

Can I have an example of the use of cstring - char arrays (with '\0' terminated). I will change string into cstring - char arrays but I don't know how to do it. It will be very helpful if i can have several examples.

(I am new to the Arduino programming and I use Arduino TIAN)

char example[] = {"01234567890123456789"};  //terminating 0 automatically added.  21 elements in the array

void setup()
{
  Serial.begin(115200);
  Serial.println(example);
  Serial.print("Length of example : ");
  Serial.println(strlen(example));
  
  example[4] = '\0';  //truncate the string after 4 characters
  Serial.print("Truncated string : ");
  Serial.println(example);
  Serial.print("Length of example now : ");
  Serial.println(strlen(example));
  
  for (int c = 0; c < 20; c++)
  {
    example[c] = c + 65;  //save ASCII value for 20 characters
  }
  example[20] = '\0';  //add terminating 0
  Serial.println(example);
  Serial.print("Length of example now : ");
  Serial.println(strlen(example));
}

void loop()
{
}