when to use the member operator struct in C code example

Example 1: struct in C

#include<stdio.h>
#include<string.h>

struct Student{
long int rollNo;
int age;
char name[50];
int totalMarks;
};

int main(){

    struct Student s1;
    //accessing members of the struct
    s1.rollNo = 1232234643;
    s1.age = 10;
    char s[]="Jonathan";
    strcpy(s1.name,s);
    s1.totalMarks = 450;

    printf("RollNumber: %d \nName: %s \nTotal Marks: %d",s1.rollNo,s1.name,s1.totalMarks);

return 0;
}

Example 2: structs in c

#include <stdio.h>
#include <stdlib.h>

struct book{ //this is like making a datatype of type book
    //these are the fields
  	char name[50];
    char author[50];
    char ISBN[11];
};

int main(){
    struct book book1; //making an instance of book called book1
    /*
    normally to store integers in a struct we can do something like
    book1.number_of_pages = 22; which is correct
    however with character arrays we need to use the strcpy
    function
    */

    strcpy(book1.name, "james and the giant tatti");
    strcpy(book1.author, "Krishan Grewal");
    strcpy(book1.ISBN, "12345678987");

    printf("book name: %s\n", book1.name);
  	printf("book author: %s\n", book1.author);
  	printf("book ISBN: %s\n", book1.ISBN);

return 0;
}