how can display a custom field under the product title and sku

Hi,

First, you can create the custom field of the table "product" and of the type "WYSIWYG" via the menu Display>Custom fields.
Then, in your products edit form, you can add text in that custom field.
Finally, in the menu Display>Views, you can edit the file product / show_default and add such code:

<?php echo $this->element->xxx; ?>

Hi,

Yes. The display in the specifications tab depends on the corresponding display setting of the custom field. So edit it via the menu Display>Custom fields and check its settings and turn off the display setting for the frontend product page.