How does insertion in an unordered_map in C++ work?

int main() 
{
    auto n=0, sockNumber=0, pairs=0;
    unordered_map<int, int> numberOfPairs;
    cin >> n;

    for(int i=0; i<n; ++i)
    {
        cin >> sockNumber;
        numberOfPairs.insert({sockNumber, 0}); // >>>>>> HERE <<<<<<
        numberOfPairs.at(sockNumber) += 1;
        if(numberOfPairs.at(sockNumber) % 2 == 0)
        {
            pairs += 1;
        }
    }

    cout << pairs;

    return 0;
}

This code counts the number of pairs in the given input and prints it. I want to know how the insert method of an unordered_map works. Every time I see a number, I've inserted it with a value '0'.

Does the insert method skip inserting the value '0' when it sees the same number again? How does it work?

Input -

9

10 20 20 10 10 30 50 10 20

Output -

3