How we insert values into database using metabox WordPress?

The following example is adding a meta box to the post edit screen and the wporg_cpt edit screen.

 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');

The wporg_custom_box_html function will hold the HTML for the meta box.

The following example is adding form elements, labels, and other HTML elements.

function wporg_custom_box_html($post)
{
    ?>
    <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="something">Something</option>
        <option value="else">Else</option>
    </select>
    <?php
}

Saving Values
The following example will save the wporg_field field value in the _wporg_meta_key meta key, which is hidden.

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');

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