Implementing a custom header via php

Daer community, I am very new to WP and am currently learning to build customized websites via WP. Right now, I am struggling with implementing a custom header showing a movie (using the “understrap“-theme).
I first installed the “understrap“-theme and then the “understrap-child“-theme an activated the latter one. Regarding to “https://codex.ww.wp.xz.cn/Custom_Headers”, I added the following lines to “understrap-child/functions.php”:

— edits to “functions.php” in “understrap-child/functions.php”/start—

register_default_headers( $headers );
$defaults = array(
	'default-image'          => get_template_directory_uri() . '/images/header.jpg',
	'width'                  => 300,
	'height'                 => 200,
	'flex-height'            => false,
	'flex-width'             => false,
	'uploads'                => true,
	'random-default'         => false,
	'header-text'            => true,
	'default-text-color'     => '',
	'wp-head-callback'       => '',
	'admin-head-callback'    => '',
	'admin-preview-callback' => '',
);
add_theme_support( 'custom-header', $defaults );

—-edits to “functions.php” in “understrap-child/functions.php”/end————-

After reloading my WP-site, under “Appearance/customize” the “Header Image” option showed up and I uploaded a respective image. Then, I copied the “header.php” from “/understrap” to “/understrap-child” and added after “</nav><!– .site-navigation –>”:

—— edits to “header.php” in “understrap-child/functions.php”/start————-
<img src="<?php header_image(); ?>" height="<?php echo get_custom_header()->height; ?>" width="<?php echo get_custom_header()->width; ?>" alt="" />
——– edits to “header.php” in “understrap-child/functions.php”/end———

… the header image shows up, but I cant change the width when changing the width from 300 to exp. 800. Then, regarding to “https://developer.ww.wp.xz.cn/themes/functionality/custom-headers/&#8221;, I added the following lines to “understrap-child/functions.php”:

——- additions to “functions.php” in “understrap-child/functions.php”/start——

function understrap_custom_header_setup() {
    $defaults = array(
        // Default Header Image to display
        'default-image'         => get_template_directory_uri() . '/images/headers/default.jpg',
        // Display the header text along with the image
        'header-text'           => false,
        // Header text color default
        'default-text-color'        => '000',
        // Header image width (in pixels)
        'width'             => 1000,
        // Header image height (in pixels)
        'height'            => 198,
        // Header image random rotation default
        'random-default'        => false,
        // Enable upload of image file in admin 
        'uploads'       => false,
        // function to be called in theme head section
        'wp-head-callback'      => 'wphead_cb',
        //  function to be called in preview page head section
        'admin-head-callback'       => 'adminhead_cb',
        // function to produce preview markup in the admin screen
        'admin-preview-callback'    => 'adminpreview_cb',
        );
}
add_action( 'after_setup_theme', 'understrap_custom_header_setup' );

—- additions to “functions.php” in “understrap-child/functions.php”/end——-

This didn’t have any effects on the width of the image either. I think, my very basic problem is that I am not experienced with WPs php-framework… but I am willing to learn…. maybe someone could help me with this??? Thx a lot!