难以理解某个函数的元素是如何工作的

我需要完全理解以下代码：

#include <iostream>
using namespace std;
double area(double length, double width);
double time(double p_area, double h_area, double mow_rate);
int main() {
    double d_plot_length, d_plot_width, d_home_side, d_mow_rate;
    double plot_area, home_area, time_taken;
    // I've used double for all of these to get the most precise values possible, something I'd only really consider doing on small programmes such as this
    cout << "What is the length of the plot? In meters please." << endl;
    cin >> d_plot_length;
    cout << "What is the width of the plot? In meters please." << endl;
    cin >> d_plot_width;
    cout<< "What is the size of the side of the house? In meters please." << endl;
    cin >> d_home_side;
    cout << "What is the rate at which you are going to be mowing? In meters per minute please" << endl;
    cin >> d_mow_rate;
    // Just getting all the data I need from the user
    plot_area = area(d_plot_length, d_plot_width);
    home_area = area(d_home_side, d_home_side);
    time_taken = time(plot_area, home_area, d_mow_rate);
    cout << "It will take " << time_taken << " minutes to mow this lawn. Better get cracking" << endl;
    return 0;
}
double area(double length, double width) {
    double value;
    value = length * width;
    return value;
}
double time(double p_area, double h_area, double mow_rate) {
    double value;
    value = (p_area - h_area) / mow_rate;
    return value;
}

我很难理解time()函数是如何工作的。

到目前为止，我了解到：

time_taken从time()函数中得到其值：time(plot_area, home_area, d_mow_rate)。

time()函数从底部的函数声明中获取其值。

double time(double p_area, double h_area, double mow_rate) {
    double value;
    value = (p_area - h_area) / mow_rate;
    return value;
}

然而，这正是我陷入困境的地方。用户被要求输入d_plot_length、d_plot_width等的值。因此，我无法理解编译器如何知道这些值p_area和h_area的实际值。

我意识到area()函数在某种程度上被用来帮助time()函数，但据我所知，time()函数中的变量P_area等没有分配值。

请有人填补我理解上的空白。

更确切地说，我想知道time_taken从流程开始到cout在屏幕上是如何显示的。就像我说的，我熟悉大多数领域，但不是所有领域。