Explanation of service example in ROS2

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I am trying to work with ROS and ROS2 for sometime now. Honestly I had zero programming experience about a month ago but i have learnt a lot in this period. Thanks to the excellent Wiki and the supporters. I have tried to understand certain concepts like shared pointers but when it comes to implementation, it gets a bit difficult for me.

Through this question i would like if someone can patiently explain each line of the following code and thus it would be more clear to me. I found the best practise as trying to answer some questions myself and i would be happy to be corrected.

add_two_ints_server

class ServerNode : public rclcpp::Node
{
public:
explicit ServerNode(const std::string & service_name) : Node("add_two_ints_server")

/* In this section, first a class ServerNode is created which inherits from the base class rclcpp::Node. An explicit constructor is called (why not implicitly ?) and the member function Node() is initialised with the name "add_two_ints_server". */

{
 // Create a callback function for when service requests are received.
auto handle_add_two_ints = 
  [this](const std::shared_ptr<rmw_request_id_t> request_header,
    const std::shared_ptr<example_interfaces::srv::AddTwoInts::Request> request,
    std::shared_ptr<example_interfaces::srv::AddTwoInts::Response> response) -> void
  {
    (void)request_header;
    RCLCPP_INFO(this->get_logger(), "Incoming request\na: %" PRId64 " b: %" PRId64,
      request->a, request->b);
    response->sum = request->a + request->b;
  };

/* Here we create a callback function and pass 3 pointer parameters, namely request_header, request and response which are taken from the serviceFile.srv. Here why an "auto" specifier is used ? Also instead of a handle, we could also use a"void service_callback()" function ? What does "this

// Create a service that will use the callback function to handle requests.
srv_ = create_service<example_interfaces::srv::AddTwoInts>(service_name, handle_add_two_ints);
}
private:
rclcpp::Service<example_interfaces::srv::AddTwoInts>::SharedPtr srv_;
};

/Here the service is created/

int main(int argc, char * argv[]) 
   {
   if (rcutils_cli_option_exist(argv, argv + argc, "-h")) {
   print_usage();
   return 0;
   }

/*What does this exactly means? */

   rclcpp::init(argc, argv);  //library initialisation call

   auto service_name = std::string("add_two_ints"); //naming the service ?
   if (rcutils_cli_option_exist(argv, argv + argc, "-s")) {
   service_name = std::string(rcutils_cli_get_option(argv, argv + argc, "-s"));
   }

/* what happens here? */

  auto node = std::make_shared<ServerNode>(service_name); //passing the service name to the node ?
  rclcpp::spin(node);

  rclcpp::shutdown();
  return 0;
  }