C ++-对象作为参数中的变量

我正在通过TheNewBostons教程学习C ++，但有一个程序我无法弄清楚它是如何工作的。在问题之前，我先给你看一下货真价实的东西。

Main.cpp:

#include <iostream>
#include "People.h"
#include "Birthday.h"


using namespace std;

int main()
{
    Birthday birthObject(12,28,1986);

    People Ob("Vidar", birthObject);
    Ob.printInfo();
}

Birthday.h：

#ifndef BIRTHDAY_H
#define BIRTHDAY_H


class Birthday
{
    public:
        Birthday(int m,int d,int y);
        void prinDate();
    private:
        int month, day, year;
};

#endif

Birthday.cpp：

#include <iostream>
#include "Birthday.h"

using namespace std;

Birthday::Birthday(int m,int d,int y)
{
    month = m;
    day = d;
    year = y;
}

void Birthday::prinDate()
{
    cout << day << "/" << month << "-" << year << endl;
}

People.h：

#ifndef PEOPLE_H
#define PEOPLE_H

#include <string>
#include "Birthday.h"
using namespace std;

class People
{
    public:
        People(string x, Birthday bo);
        void printInfo();
    private:
        string name;
        Birthday dateOfBirth;
};

#endif

People.cpp：

#include "People.h"
#include "Birthday.h"

#include <iostream>
#include <string>
using namespace std;

People::People(string x, Birthday bo)
: name(x), dateOfBirth(bo)
{

}

void People::printInfo()
{
    cout << name << " was born on ";
    dateOfBirth.prinDate();
}

我不知道的事情是如何将对象用作变量和参数，以及如何在不调用构造函数的情况下创建对象（在People.h中）。