For ("\0".."👍").to_a https://github.com/ruby/ruby/blob/f114089585d486920f53c0c4cb92d73dc4ed577c/string.c#L4246-L4326

string.c

static VALUE
str_upto_each(VALUE beg, VALUE end, int excl, int (*each)(VALUE, VALUE), VALUE arg)
{
    /* (snip) */

    /* normal case */
    n = rb_str_cmp(beg, end);
    if (n > 0 || (excl && n == 0)) return beg;

    after_end = rb_funcallv(end, succ, 0, 0);
    current = rb_str_dup(beg);
    while (!rb_str_equal(current, after_end)) {
        VALUE next = Qnil;
        if (excl || !rb_str_equal(current, end))
            next = rb_funcallv(current, succ, 0, 0);
        if ((*each)(current, arg)) break;
        if (NIL_P(next)) break;
        current = next;
        StringValue(current);
        if (excl && rb_str_equal(current, end)) break;
        if (RSTRING_LEN(current) > RSTRING_LEN(end) || RSTRING_LEN(current) == 0)
            break;
    }

    return beg;
}