Creating a custom Meta Box on Backend in Wordpress

// register meta box
function meta_fields_add_meta_box(){
    add_meta_box(
        'meta_fields_meta_box', 
        __( '<span style="color:red;">Course Booking URL</span>', 'textdomain' ), 
        'meta_fields_build_meta_box_callback', 
        'page', 
        'side',
        'default',
        // hide the meta box in Gutenberg
        array('__back_compat_meta_box' => true)
    );
}
add_action( 'add_meta_boxes', 'meta_fields_add_meta_box' );

// build meta box content
function meta_fields_build_meta_box_callback( $post ){
    wp_nonce_field( 'meta_fields_save_meta_box_data', 'meta_fields_nonce' );

    $paymenturl = get_post_meta( $post->ID, '_meta_fields_payment_url', true );
    $courseid   = get_post_meta( $post->ID, '_meta_fields_course_id', true );


   ?>
   	<table id="courseurlandid">
	    <tr>
	        <td>
	            <label for="meta_fields_payment_url">Payment URL:</label>
	        </td>
	        <td>
	            <input type="text" id="meta_fields_payment_url" name="meta_fields_payment_url" value="<?php echo esc_attr( $paymenturl ); ?>" class="meta-field-input" />
	        </td>
	    </tr>
	    <tr>
	        <td>
	            <label for="meta_fields_course_id">Course ID:</label>
	        </td>
	        <td>
	            <input type="text" id="meta_fields_course_id" name="meta_fields_course_id" value="<?php echo esc_attr( $courseid ); ?>" class="meta-field-input" />
	        
	        </td> 
	    </tr>
	</table>

   <?php
}

// save meta box data
function meta_fields_save_meta_box_data( $post_id ){
    if ( ! isset( $_POST['meta_fields_payment_url'] ) )
        return;

    if ( ! isset( $_POST['meta_fields_course_id'] ) )
        return;

    if ( ! wp_verify_nonce( $_POST['meta_fields_nonce'], 'meta_fields_save_meta_box_data' ) )
        return;

    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
        return;

    if ( ! current_user_can( 'edit_post', $post_id ) )
        return;

	$paymenturl = sanitize_text_field( $_POST['meta_fields_payment_url'] );
	$courseid = sanitize_text_field( $_POST['meta_fields_course_id'] );

	$paymenturl_old = get_post_meta( $post_id, '_meta_fields_payment_url', true );
	$courseid_old = get_post_meta( $post_id, '_meta_fields_course_id', true );

	if (empty($paymenturl_old) || empty($courseid_old)) {
	    update_post_meta( $post_id, '_meta_fields_payment_url', $paymenturl );
	    update_post_meta( $post_id, '_meta_fields_course_id', $courseid );
	}

}
add_action( 'save_post', 'meta_fields_save_meta_box_data' );

A meta box gets created on a post type page in the side and shows 2 fields and values from those 2 fields would be saved in post meta once the page is saved