How add metabox to your WordPress posts. Without plugin

A standard example of creating a text and select metabox with multiple security checks

Create in yours theme directory file advanced-post-widget.php

And add to it code below:

<?php

function expand_my_post_metabox() {

    add_meta_box('expand_post_rating', esc_html__('Expand Post Settings', 'clean'), 'expand_post_html', 'post', 'normal', 'high');

    // expan_post_rating - metabox id
    // esc_html__('Expand Post Settings', 'clean') - translation ready metabox area name
    // expand_post_html - function for output html
    // post - type of post for which we set the metabox
    // normal - metabox width
    // high - metabox priority
}

add_action('add_meta_boxes', 'expand_my_post_metabox');

function expand_post_html($post) {

    $rating_value = get_post_meta($post->ID, 'expand_rating_value', true);
    $expand_post_type = get_post_meta($post->ID, 'expand_post_type', true);
	
    
wp_nonce_field('noncename111', 'expand_my_post_metabox_verify');
    
    ?>

<p>
    <lebel for="expand_rating_value"><?php esc_html_e('Post Rating', 'clean'); ?></lebel>
    <input type="text" id="expand_rating_value" name="expand_rating_value"
        value="<?php echo esc_attr($rating_value); ?>">
</p>

<p>
    <lebel for="expand_post_type"><?php esc_html_e('Post Type', 'clean'); ?></lebel>
    <select id="expand_post_type" name="expand_post_type">
        <option value=""><?php esc_html_e('Select Post type', 'clean'); ?></option>
        <option value="review" <?php if($expand_post_type == 'review'){echo 'selected';}  ?>>
            <?php esc_html_e('Review', 'clean'); ?>
        </option>
        <option value="interview" <?php if($expand_post_type == 'interview'){echo 'selected';}  ?>>
            <?php esc_html_e('Interview', 'clean'); ?>
        </option>
        <option value="guide" <?php if($expand_post_type == 'guide'){echo 'selected';}  ?>>
            <?php esc_html_e('Guide', 'clean'); ?>
        </option>
    </select>
</p>


<?php

}

function expand_my_post_metabox_save($post_id, $post) {

    if(!isset($_POST['expand_my_post_metabox_verify']) || !wp_verify_nonce($_POST['expand_my_post_metabox_verify'], 'noncename111' )) {
        return $post_id;
    }

    if(defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
        return $post_id;
    }

    if($post->post_type !='post') {
        return $post_id; 
    }
    
    $post_type = get_post_type_object($post->post_type);
    if(!current_user_can($post_type->cap->edit_post, $post_id)) {
        return $post_id;
    }
    
    if(isset($_POST['expand_rating_value'])) {
        update_post_meta($post_id, 'expand_rating_value', sanitize_text_field($_POST['expand_rating_value']));
    } else {
        delete_post_meta($post_id, 'expand_post_type',);
    }

    if(isset($_POST['expand_post_type'])) {
        update_post_meta($post_id, 'expand_post_type', sanitize_text_field($_POST['expand_post_type']));
    } else {
        delete_post_meta($post_id, 'expand_post_type',);
    }
    
    return $post_id;
    
}

add_action('save_post', 'expand_my_post_metabox_save', 10, 2);

?>

Attach your metabox to a file function.php with code

require get_template_directory() . '/advanced-post-widget.php';

Modify metabox to suit your needs. Good luck