big number operation problem

[qaz1423078740]

I would like to ask how BIG data is represented, is it array addition?
For example, {254,1} means 255. Also, is there an upper limit for a single element of array?
I tried to add two big numbers 2147483645 to show what is the value of -6, I think there is no carry operation, I don't understand. Here is the test code I did

BIG a;
BIG b;
BIG d;
BIG_one(a);
BIG_one(b);
int c = 2147483644;

BIG_inc(a, c);
BIG_inc(b, c);
// BIG_toBytes(b,a);
BIG_add(d, a, b);
for (int i = 0; i < sizeof(d) / sizeof(d[0]); i++) {
Serial.println(d[i]);
Serial.println(" ");
}

-6 means ?
Is this modification correct?

BIG a;
BIG b;
BIG d;
BIG_one(a);
BIG_one(b);
int c = 2147483644;

BIG_inc(a, c);
BIG_norm(a);
BIG_inc(b, c);
BIG_norm(b);

BIG_add(d, a, b);

for (int i = 0; i < sizeof(d) / sizeof(d[0]); i++) {
Serial.print(d[i]);
Serial.println(" ");
}

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