I have a big number stored in a string and try to extract a single digit. But what are the differences between those calls?

#include <iostream>
#include <string>

int main(){
    std::string bigNumber = "93485720394857230";
    char tmp = bigNumber.at(5);
    int digit = atoi(&tmp);
    int digit2 = atoi(&bigNumber.at(5))
    int digit3 = atoi(&bigNumber.at(12));
    std::cout << "digit: " << digit << std::endl;
    std::cout << "digit2: " << digit2 << std::endl;
    std::cout << "digit3: " << digit3 << std::endl;
}

This will produce the following output.

digit: 7

digit2: 2147483647

digit3: 57230

The first one is the desired result. The second one seems to me to be a random number, which I cannot find in the string. The third one is the end of the string, but not just a single digit as I expected, but up from the 12th index to the end of the string. Can somebody explain the different outputs to me?

EDIT: Would this be an acceptable solution?

char tmp[2] = {bigNumber.at(5), '\0'};
int digit = atoi(tmp);
std::cout << "digit: " << digit << std::endl;