Custom Meta Boxes for post (ok)

https://developer.wordpress.org/plugins/metadata/custom-meta-boxes/

C:\xampp\htdocs\wordpress\wp-content\themes\twentytwentyone\functions.php

<?php  
function wporg_add_custom_box() {
    $screens = [ 'post', 'wporg_cpt' ];
    foreach ( $screens as $screen ) {
        add_meta_box(
            'wporg_box_id',                 // Unique ID
            'Custom Meta Box Title',      // Box title
            'wporg_custom_box_html',  // Content callback, must be of type callable
            $screen                            // Post type
        );
    }
}
add_action( 'add_meta_boxes', 'wporg_add_custom_box' );
function wporg_custom_box_html( $post ) {
    $value = get_post_meta( $post->ID, '_wporg_meta_key', true );
    ?>
    <label for="wporg_field">Description for this field</label>
    <select name="wporg_field" id="wporg_field" class="postbox">
        <option value="">Select something...</option>
        <option value="aaaaaaaaaaagggggggggghhhhhhhhhh" <?php selected( $value, 'aaaaaaaaaaagggggggggghhhhhhhhhh' ); ?>>aaaaaaaaaaagggggggggghhhhhhhhhh</option>
        <option value="else" <?php selected( $value, 'else' ); ?>>Else</option>
    </select>
    <?php
}
function wporg_save_postdata( $post_id ) {
    if ( array_key_exists( 'wporg_field', $_POST ) ) {
        update_post_meta(
            $post_id,
            '_wporg_meta_key',
            $_POST['wporg_field']
        );
    }
}
add_action( 'save_post', 'wporg_save_postdata' );
function wporg_meta_box_scripts() {
     // Get current admin screen, or null.
    $screen = get_current_screen();
    // Verify admin screen object before use.
    if ( is_object( $screen ) ) {
        // Enqueue only for specific post types.
        if ( in_array( $screen->post_type, [ 'post', 'wporg_cpt' ], true ) ) {
            wp_enqueue_script( 'wporg_meta_box_script', plugin_dir_url( __FILE__ ) . 'admin/meta-boxes/js/admin.js', [ 'jquery' ], '1.0.0', true );
            wp_localize_script(
                'wporg_meta_box_script',
                'wporg_meta_box_obj',
                [
                    'url' => admin_url( 'admin-ajax.php' ),
                ]
            );
        }
    }
}
add_action( 'admin_enqueue_scripts', 'wporg_meta_box_scripts' );
?>