Undefined behavior when converting char to std::string c++

My program takes in a vector std::vector<std::string> vector and a character char separator and returns a string with all of the strings added together between the separator character. The concept is: vector[0] + separator + vector[1] + separator

The Code

std::string VectorToString(std::vector<std::string> vector, char separator)
{
    std::string output;
    for(std::string segment : vector)
    {  
        std::string separator_string(&separator);
        output += segment + separator_string;
    }
    return output;
}

int main()
{
    std::vector<std::string> vector = {"Hello",  "my",  "beautiful", "people"};
    std::cout << VectorToString(vector, ' ');
}

My expected output is Hello my beautiful people However the output is:

Hello �����my �����beautiful �����people �����

What I have found is that something is wrong with the character, specifically its pointer: std::cout << &separator; -> �ƚ��. However if I do like this: std::cout << (void*) &separator; -> 0x7ffee16d35f7. Though I don't really know what (void*) does.

Question:

1.What is happening?

2.Why is it happening?

3.How do I fix it?

4.How do I prevent it from happening in future projekts?