Creating navigation bar troubles

Hello,

I want to fully understand how nav bars work in wp.

I had put in my function.php

<?php 

function register_my_menus() {
  register_nav_menus(
    array(
      'header-menu' => __( 'Header Menu' ),
	  'nav-bar' => __( 'nav-bar' ),
	  'Menu1' => __( 'Menu1' )
	  // ,'extra-menu' => __( 'Extra Menu' )
    )
  );
}
add_action( 'init', 'register_my_menus' );

?>

To register my additional 2 menus. “nav-bar” and “Menu1”. I don’t know where does come from -> Header menu, if I dont have such menu name in my Menu in Appereance tab.

My code in header.php is:

<?php wp_nav_menu( array(
						'menu_class'      => 'menu',
						'container_class' => '',
						'menu' => 'nav-bar'
 )

 ); ?>

1) I set containter_class to null and menu_class to ‘menu’ and I have div class="menu-menu1-container" name instead of my div class name ‘menu’ – why?
2) If I set in header.php any value to ‘menu’ it showing my Menu1 content – why?
3) If I set 'theme_location' => 'header-menu' in header.php I get content from ‘nav-bar’ and div name ‘menu’ – why? Where does it know which menu and this correct name?

Thank you for sharing your help!

@@EDIT
To point 3) It showing all my sites, not only from nav-bar.