Variable Number of Arguments - General Questions

Which header provides va_list, va_start, va_arg and va_end?

What will be the output of the program?

#include <stdio.h>
#include <stdarg.h>
int maxof(int n, ...) {
    va_list ap;
    int i, m, v;
    va_start(ap, n);
    m = va_arg(ap, int);
    for (i = 1; i < n; i++) { v = va_arg(ap, int); if (v > m) m = v; }
    va_end(ap);
    return m;
}
int main(void) {
    printf("%d\n", maxof(4, 3, 9, 2, 7));
    return 0;
}

Which macro starts processing of variable arguments?

Why must a variadic function have at least one named parameter (in C before C23)?

How does a variadic function know how many arguments it received?