I am trying to iterate over characters in c and really struggling to understand how to do it. I have this code which does that, however i do not completely understand it (very new to coding and C, currently taking the cs50 course).

#include <cs50.h>
#include <ctype.h>
#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[])
{
    // Check if there is exactly one command-line argument
    if (argc != 2)
    {
        printf("This is an invalid keyn");
        return 1;
    }

    // single command-line argument, case insensitive, 26 length, all letters of the alphabet only once.
    int key_length = strlen(argv[1]);

    // checks the length of the key to make sure it is 26 characters long
    if (key_length != 26)
    {
        printf("Your key must contain 26 lettersn");
        return 1;
    }

    // check if every letter of the alphabet is used.
    for (int i = 0; i < key_length; i++)
    {
        char c = argv[1][i];
        if (!isalpha(c))
        {
            printf("Your key must be alphabeticn");
            return 1;
        }
    }

    // Convert the key to uppercase and store it back in argv[1]
    for (int i = 0; i < key_length; i++)
    {
        argv[1][i] = tolower(argv[1][i]);
    }

    //prompt user for plaintext
    string plain_text = get_string("Plaintext: ");

    //output ciphered text as ciphertext (currently not implemented)

    return 0;
}

I have tried a few videos and articles but struggling to understand it if anyone can explain it /point me to good resources for it.