This is how to add metabox in WordPress page or post

mwp-metabox.php

<?php
/*
check out the full tuts: https://www.ministryofwp.com/how-to-add-wordpress-meta-box/
*/

function add_guest_details() {
     
  add_meta_box('guest-details', 'Guest details', 'guestcallback', 'post', 'side', 'high');
   
}
 
add_action('add_meta_boxes', 'add_guest_details');

function guestcallback($post) {
     
      wp_nonce_field( 'guest_meta_box', 'guest_nonce' );
      
      $name_value = get_post_meta( $post->ID, '_guest_name', true );
      $link_value = get_post_meta( $post->ID, '_guest_link', true );
      
      echo '<label for="guest-name">'. 'Guest Blogger Name' .'</label>';
      echo '<input type="text" id="guest-name" name="guest_name" placeholder="Example" value="'.$name_value.'" size="25"/>';
      echo '<p class="howto">'. 'Add the name of the Guest' .'</p>';
      
      echo '<label for="guest-link">'. 'Guest website Link' .'</label>';
      echo '<input type="text" id="guest-link" name="guest_link" placeholder="http://www.example.com/" value="'.$link_value.'" size="25"/>';
      echo '<p class="howto">'. 'Add the link of the website' .'</p>';
   
}

function save_guest( $post_id ) {
     
    if ( ! current_user_can( 'edit_post', $post_id ) ) {
        return;
    }
 
    if ( ! isset( $_POST['guest_nonce'] ) ) {
        return;
    }
 
    if ( ! wp_verify_nonce( $_POST['guest_nonce'], 'guest_meta_box' ) ) {
        return;
    }
    
   if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
        return;
    }
 
    if ( ! isset( $_POST['guest_name'] ) || ! isset( $_POST['guest_link'] ) ) {
        return;
    }
 
    $guest_name = sanitize_text_field( $_POST['guest_name'] );
    $guest_link = sanitize_text_field( $_POST['guest_link'] );
 
    update_post_meta( $post_id, '_guest_name', $guest_name );
    update_post_meta( $post_id, '_guest_link', $guest_link );
 
}
 
add_action( 'save_post', 'save_guest' );

/*to show metabox value in post or page*/
$name_value = get_post_meta( $post->ID, '_guest_name', true );
                    $link_value = get_post_meta( $post->ID, '_guest_link', true );
                    echo ''.$name_value.'<br/>'; 
                    echo $link_value;