How do malloc() and free() work?

I want to know how malloc and free work.

int main()
{

    unsigned char *p = (unsigned char*)malloc(4*sizeof(unsigned char));
    memset(p,0,4);
    strcpy((char*)p,"abcdabcd"); // **deliberately storing 8bytes**
    cout << p;
    free(p); // Obvious Crash, but I need how it works and why crash.
    cout << p;
    return 0;
}

I would be really grateful if the answer is in depth at memory level, if it’s possible.

How do malloc() and free() work?