How to save the webform data to custom table?
You can add custom submit handler to your Webform through hook_form_alter for inserting it's data into your custom table.

To achieve it check the below example.

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Database\Database;

function MODULE_NAME_form_alter(&$form, $form_state, $form_id){
  if($form['#form_id'] == "YOUR_WEBFORM"){
    $form['actions']['submit']['#submit'][] = 'YOUR_CUSTOM_FUNCTION';
  }
}

function YOUR_CUSTOM_FUNCTION(&$form, $form_state){
  $query = Database::getConnection();
  $query->insert('YOUR_CUSTOM_TABLE_NAME')->fields(
    array(
      'FIELD_NAME_IN_TABLE' => $form_state->getValue('FIELD_NAME_IN_FORM'),
    )
  )->execute();
}