confused by output of this c code, also the execution result of "myprofile"

 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>


 int main() {
     char buf[15];
     char *buf1 = "CE_and_IST";
     char buf2 = 'z';
     int fd = open("myfile", O_RDWR);
     lseek(fd, 10, SEEK_SET);
     int n = read(fd, buf, 10);
     printf("%d\n", n);
     n = write(fd, (const void *)buf1, 10);
     printf("%s\n", buf);
     lseek(fd, 0, SEEK_SET);
     close(1); dup(fd); close(fd);
     n = write(1, (const void *)&buf2, 1);
     close(1);
     return 0;
     }

and the contents of "myprofile" is Welcome_to_CIS

so the output for this code is, and I'm really confused, is 5 come from 15 minus 10, I thought the length of the code is 15. like what does exactly this code do?

5
_CIS

and the contents of "myprofile" is after execution is

Zelcome_to_CIS
CE_and_IST

How did they get to this?