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Hi..

you have right way about "displayAdminProductsExtra" call the your module file this hook like this 

public function hookDisplayAdminProductsExtra($params) {
	return $this->context->smarty->fetch($this->local_path . 'views/templates/hook/displayAdminProductsExtra.tpl');//this is path of your module display file
}

and "displayAdminProductsExtra.tpl" file somthing your html code

<input type="text" name="input" value="demo" />

 about save the this fields use this hook "actionProductUpdate"

public function hookActionProductUpdate($params) {
	$id_product = $params['id_product'];
	/// your custom code here
}

Thanks